What is it called when you say the wrong word..?

I forgot the term. It's not irony nor sarcasm and it's not by accident. There was once an English woman who always said the wrong word when she meant the definition of another word, but she didn't know that. I believe the term was named after her. An example would be like; " He's so... show more I forgot the term. It's not irony nor sarcasm and it's not by accident. There was once an English woman who always said the wrong word when she meant the definition of another word, but she didn't know that. I believe the term was named after her. An example would be like; " He's so desolate" The correct word would be "desperate", but the user applied its definition to "desolate" (They didn't know that).

What is the term when one uses a (wrong) word when they mean the definition of another?
Update: It's not a homonym. It is an actual term, but it's not regularly used. My old English teacher once told me the term, but now I forgot it.
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