Tart is a British slang term for a promiscuous person with fairly low standards. It is most commonly, but not exclusively, used to describe a woman of this ilk.
The origin of Tart in this usage is that it is an abbreviation of “Sweetheart”.
A Tart will sleep around with plenty of colleagues, particularly at the office party, slowly doing the rounds with anyone who is interested in the fullness of time.
Tart is also used as a synonym for Slapper and Scrubber.
“That new guy is a right old tart, he’s already shagged half the typing pool!”.