i.g.t.s. is the acronym for Involuntary Groin Thrusting Syndrome (a sudden burst of humping convulsions), it is not actually involuntary but is in fact done for a good laugh in public places. best if used on random people because then you can then explain that you have a disease and get away with humping ridiculously beautiful women or you can piss off ridiculously stupid looking guys.
man: (i.g.t.s.'s his friend next to him)
random person: "dude, stop that"
man: sorry, i have i.g.t.s.