How much money do you brits mean when you refer to "a quid"? And more importantly, why do you use that term?

A quid is one pound.
It's a slang term but the origins are obscure. What is know is that it has been in use for around four hundred years. I don't have time to research at the moment but if I get any firm info. later, I'll add it.

Vescere bracis meis.

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No-one really knows where the slang word "quid", meaning a Pound Sterling, comes from. See The Word Detective.

It is unusual in that the singular and plural are the same; we talk about "one quid" or "five quid" -- not "five quids".

However, a common informal idiom is to talk about "being quids in", meaning making a profit, and there the word has an s on the end. Go figure.

A quid is a pound sterling. The word is not usually used in the plural - it's ten quid, not ten quids.

I don't know why we use it any more than I know why you use the term "buck" for a dollar. I suspect both terms are quite old.

We would only use it as part of a sentence or expression that conveys some extra meaning, for example, "...fifty quid and it's yours..." We would not say, for example, "...how many quid...?"

I suspect this is the same as in the US where you might say, "...That'll be 50 bucks..." but you wouldn't say, "...how many bucks is that...?"

And the expression I've used as the title to this piece? That is a slang term meaning "...You'll do well out of a course of action..." For example, "...If you buy oil shares right now you'll be quids in once the threat of war's over..." This is one time when the word is used in the plural.

Richard English

Quid is extremely common here. My friend went a phrase of saying squid instead, which is logical if your rounds often added up to £6.