In Reply to: You are a one posted by ESC on February 02, 2009 at 19:50:
: : Can anyone tell me why people say - 'you are a one'?
: Could we have more context? There are several ways to go with this. If may refer to a people rating system. Good: You're A-No. 1 -- the best as in A-1. See https://www.phrases.org.uk/meanings/21500.html Or "one singular sensation..." as in "One" from A Chorus Line. Or not so good, based on the rating system in the movie "10."
The way the phrase is used in the UK (and an idea of age, if not exactly derivation):
You are a one! = You are droll! Probably since around 1880; mainly Cockney. In modern usage it is often said in an affected or mockingly imitative manner.