ISF is Instant Skullf***, one of the best competitive Left4Dead teams in PC Gaming. They've performed in Newegg tournaments, Alienware, and gXl. Ranked in the top ten of Left4Dead teams in competition.
by sindrix December 11, 2009
Bobby Chong is looking to buy 2 doughnuts for 50 cents each. The cashier lady says that will be; a dollar seven total. Bobby Chong whips out the B.C(Bank Card), swipes, punches in the pin and waits for the thumbs up for approval then 3 letters in Capitals show up on the machine; ISF. Cashier shakes her head in shame and pitty. Bobby curses and walks away for the counter doughnutless; and realizes that he will never step foot in another Tim Hortons again due to ISF...$1.07 damn. How embarrassing.
by JNL January 12, 2005
icky sexual feeling; the kind of feeling you get when something makes you uncomfortable to think about or see, i.e. incest, your English teacher naked, etc.
That creepy guy who works with me looked me up and down and said, "Nice shirt." and I got total ISF.
by Jocelynnn September 23, 2005
ISF means "instant skullfuck". To isf someone is to pwn or destroy someone completely, such as in day to day life, sports, or in video games.
"We are going to ISF that that team later in dodgeball!"
"My girlfriend ISFed me earlier when she dumped me!"
"My girlfriend ISFed me earlier when she dumped me!"
by youmadbra December 11, 2009