Origins unknown; used as a codeword to describe people hooking up; now used as common slang in (mostly the north of) Scotland to describe any kind of sexual activity, and indeed many other activities which have little or nothing to do with sex, and is generally used to indicate that something fortunate has occurred. It can be used as a legitimate verb in regular conversation, but it is normally just shouted, and is accompanied by a hand action where one puts the thumb and middle finger together, and taps them with the index finger.
Chris and Eleanor start making out at a party.
Chris's friends: "PIECING!"
Chris and Eleanor start making out in a library or a similarly quiet/awkward situation.
Chris's friends: -hand action-
Chris: I got the new Call of Duty!
Chris's friends: "PIECING!" -hand action-
Chris: I pieced four girls last night!