Dylan Edwards (asker) Jul 20, 2009:

Thank you for the helpful comments. 'Bighead' was used quite a lot by schoolchildren during my schooldays.

Sokratis VAVILIS Jul 20, 2009:

some clarifications... When someone says for a person "την έχει ψωνίσει" , it means he is mentally absent, he is nuts (another way to express this is : του λασκάρισε η βίδα). However, always depending on the broader context "την έχει ψωνίσει" can mean that someone reacts illogically /takes illogical decisions.
Saying that someone is "ψώνιο" or "ψωνάρα" means that his acts and decisions are far away different compared to the ones a reasonable person takes; in that case, bighead (UK), swellhead (US), show-off, crackpot express the idea.

Dylan Edwards (asker) Jul 19, 2009:

Thanks all - A friend, who returned from Greece recently, explained that ψωνάρα means (roughly) an empty-headed person, but then she modified this by saying that really it means a person who is mentally "absent". I'm really not sure which meaning is uppermost: bighead/swellhead, or mentally "not quite there".