It's mathematics slang for cutie pie. Seeing as 3.14 is the value of pi, well actually 3.141593 is probably closer even though it's an infinite number so 3.14 will do, but yeah you can catch my drift.
Usually in a text message or written/typed message of some sort:
Guy 1: Did you see andrea?
Guy 2: Yeahhh! She's a QT 3.14
Guy 3: lol i see what you mean. lol.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi