Apparently its the highest of high promises between friends and basicly "to five-it" is "to promise"
Person 1: I heard Bob talking trash about you.
Person 2: Seriously?
Person 1: yah.
Person 2: five-it?
Person 1: five-it.
Person 2: Seriously?
Person 1: yah.
Person 2: five-it?
Person 1: five-it.
by Ginanmaca May 09, 2006