Usually used by a third party to describe the unusual, changed behaviour of a friend who has fallen head-over-heels in love, and is celebrating that event with prodigious feats of erotic endurance. The third party is, by definition, jealous, and the protagonist is in unashamed and glorious celebration of his new-found partner in outrageous lust . . .
He's had his curtains closed all weekend; the shop hasn't opened - I reckon he's completely cuntstruck with that dirty bint from Machynlleth . . .