Prat

My spellchecker rejected the word prat when I used it to refer to myself as a fool, ass or idiot. So I checked the dictionary and yes, it does mean that though I suppose it still qualifies as slang. Hence its omission from the spellchecker’s dictionary.

It derives from an Old English word for a prank or a trick.

What I had not realised was that it also means ‘a detail’ or ‘a member of a collective unit’. So it is defined in the glossary of chabad.org – a website devoted to utilizing internet technology to unite Jews worldwide, empower them with knowledge of their 3,300 year old tradition and foster within them a deeper connection to Judaism’s rituals and faith.

It seems that it is also an obsolete slang word for the buttocks - politely known as the 'fleshy part of the human body that you sit on'. I have never heard it used as that. Or as a slang word for the female genitals which is given as another alternative meaning.