A Reference to Metal Gear solid 3: snake eater. the quote is usually associated with a sequence where the theme of the game plays durring a two minuet sequence of climbing a ladder. this quote being the very first line of the song, and that being the beggining of the infamous ladder climb, climbing ladders and uttering the quote are often seen together within the company of metal gear fans.
To engage in penetrative genito-genital or genito-anal sexual intercourse to climax three times, in fairly close succession (such that it can be considered one session or takes place over one night). Not necessarily interpreted literally, but rather used figuratively to emphasize the relative hot-ness of the act itself or the person(s) involved, i.e. not once, not twice, but thrice.
E.g.: "My God, I would thrice lay him!" or "You totally thrice laid my ex-..."
Originally appears in an exchange between Gandhi and Geldemore of Clone High - S01E09-- Geldemore: "... As a reward for your bravery and leadership, I shall thrice lay you." Gandhi: "Uhhh, that's—, that's cool. But {cough} I'm good."