It means "natural airhead" or simply 'airhead', a person prone to making silly mistakes.
The word 'tennen' is an abbreviation for 'tennen boke', which is usually translated as 'natural airhead'. The word "tennen" originally just meant 'nature'/'natural', 'spontaneity'/'spontaneous'. The word 'boke' means 'stupid', 'silly', 'idiot', 'fool'. You may have heard it in the context of Japanese comedy. In Japanese comedy duos the 'straight man' is called 'tsukkomi' and the 'funny man' is called 'boke', who is usually at the receiving end of the jokes.
So, tennen (boke) means 'natural airhead'. I've also seen it translated simply as 'airhead'. I think 'stupid' would be a little too strong of a translation. It just means that the person can be a little scatterbrained sometimes.