To lightly rub the tips of your fingers across someone's back (or anywhere else for that matter) to produce a slight tickle that feels very relaxing and can put you to sleep.
Guy 1: I was having a hard time sleeping, so my girlfriend gave me tickies. It was the most relaxing thing ever.
stoner 1:yo u got five?
stoner 2: yeah
stoner 1: yo u wanna go eat twinkies?
stoner 2: HELL YEAH!!!
dumb parent: haha you crazy kids and your inside jokes