An exclamation. 12:15 is the ultimate argument-ender, however it only works if both "ya face" and "ya mum" have been mentioned in the argument.
exception: Saying 12:15 to someone at exactly a quarter-past noon/midnight grants you instant victory before an argument has even begun.

12:15, or 12:15 tits are tits that have nipples facing in different directions
It is referred to as 12:15 because the tits that inspired the term had one nipple that pointed up and one to the side, giving the impression that the nipples were like the little hands from a clock, and if it was a clock, it would read 12:15