I don't see it in any normal dictionaries, but I did see it here
trepidacious (or trepidatious)
(overwrought word for) trepid
(How is that for an obscure definition?)
But...I think it is not a word--or is it? I'm afraid you and your friend will have to continue arguing. Here's a diatribe about it:
Part 1. The main story. I begin with a posting I made to the American Dialect Society mailing list on November 2, somewhat revised here.
While I was putting Robert Hartwell Fiske's The Dictionary of Disagreeable English: A Curmudgeon's Compendium of Excruciatingly Correct Grammar (2005 -- yes, 2005, this book is really on the cutting edge of the time line) onto the shelf, it fell open to a page with an entry for TREPIDACIOUS, which caught my eye because i am an occasional (and proud) user of the word TREPIDATIOUS 'tremblingly reluctant' and took TREPIDACIOUS to be a misspelling of this word, which should have a T because TREPIDATION does. (A quick web Google search showed ca. 2,150 hits for TREPIDATIOUS, to 658 for TREPIDACIOUS, and Google asked about the latter if I meant the former. The site wordsmith.org notes the latter spelling and suggests that the word should be spelled with a T "if at all" -- on which, see below.) In any case, from here on I'm referring to the item in question as trepidatious; spelling isn't the issue.
Fiske's entry declares sternly that trepidatious is "solecistic for fearful (and similar words)"; he offers uneasy and anxious as well as fearful. A bit of thesaurisizing for the noun trepidation provided the following alternatives to trepidatious: agitated, alarmed, anxious, apprehensive, dismayed, fearful, frightened, hesitant, reluctant, timid, uneasy. But none of these expresses the shade of meaning I want when I use trepidatious; I want the sense of trembling reluctance that trepidation conveys. Trepidatious is simply a more vivid adjective than all the alternatives (though apprehensive comes closest to the effect I want), certainly a better choice than the three blander options that Fiske provides. On the general principle that you should use the best word for your purposes, I choose trepidatious.
Ah, but Fiske doesn't allow me this choice. He asserts, with utter self-assurance and no qualification:
Trepidacious is not a word.
adding that "Trepidation, meaning fear or apprehension, is a word, as as trepid (the antonym of the more familiar intrepid), meaning timid or fearful." (Yeah, like I'm going to use trepid. Even Fiske doesn't go so far as to advise that I use trepid instead of trepidatious.)
I've been hearing this "not a word" bullsh*t since I was a kid, usually applied to non-standard ain't and taboo **** (neither of which Fiske bothers to inveigh against, undoubtedly because they're so far beyond the pale). It mystified me then, and it angers me now. It's (literally) superhyperbolic, two steps of exaggeration beyond reality, and it's insulting.
First, the reality (and the insult): The admonition that people of taste and refinement should not use X. This is an expression of the admonisher's judgment about linguistic usages, couched as an injunction. It's insulting because the admonisher takes himself to be the arbiter of other people's behavior and brooks no objection that people of taste and refinement do in fact use X. The admonisher knows what's right; it's not a matter for discussion. Well, I'm a person of some taste and refinement (in the appropriate circumstances), and I use trepidatious. Stop telling me I'm a clumsy ignoramus.
A side issue here. I assume that Fiske objects to trepidatious because it's a recent innovation: "Even though people use it (horrible to hear, ridiculous to read though it is), no major dictionary, remarkably, has yet included trepidacious in its listing." Give them time, Fiske, give them time. The word has a lot going for it, beyond the fact that some careful writers -- like me -- use it. It's an instance of a small but significant pattern in English derivational morphology: words in -atious meaning 'inclined to -ation '. Ostentatious, flirtatious, disputatious, vexatious. Trepidatious is transparent, easily understood. It's a good thing to have. (Trepid, in contrast, is a dead loser.)
But back to superhyperbole. We start with the admonition that people of taste and refinement should not use X. This is then exaggerated, elevated to the admonition that people, in general, should not use X; what should govern the behavior of the "best" of us (those are genuine sneer quotes) in certain circumstances should govern the behavior of all of us, all of the time, in all contexts, for all purposes. (What a remarkable lack of nuance! What a divorcement from the complex textures of social life!)
As if that weren't enough, it ratchets up, hysterically, one more notch, to the bald assertion that X simply isn't available for use; it's just not part of the social repertoire. My dear, it just isn't done.
But if it truly isn't done, then there's no need for the admonitions.
Don't tell me there's "no such word". Parade your idiosyncratic prejudices, if you wish, and if your mind is open enough we might be able to talk about the bases of your prejudices (and mine). But don't lie to me about the state of the language.
Part 2. The coda. It turns out, contrary to widespread belief in certain circles, that Fiske is not entirely a write-only subscriber to ADS-L. On November 15, Fiske posted a brief message entitled "Arnold Zwicky et al. aside ...", which suggests that he had noticed my posting. There are two parts to this message: first, an excerpt from a review of DDE:
However curmudgeonly, Mr. Fiske betrays a bluff humanitarian spirit. ... [Fiske] wants to save [the English language]. And he knows that he can count on little help. Dictionaries "have virtually no standards, offer scant guidance, and advance only misunderstanding." His own flogging of Merriam-Webster's is one of the many pleasures of this lovely, sour, virtuous book. -- Erich Eichman in Wall Street Journal (Nov. 12)
And, second, a blurb, presumably in his own voice:
The Dictionary of Disagreeable English -- it's an annoying, amusing book.
Cute. He cops to "sour" and "annoying", but it's all in the service of the very salvation of the English language. Who could argue with that? And it's humanitarian labor to boot; I mean, the Wall Street Journal says so! The man must be not only a savior, but a saint, working so hard for human welfare and social reform.
Me, I'd be more than a bit trepidatious about having him at the helm of the Ministry of Language.