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.
aria is one of the sweetest mutuals ever. her edits are amazing and when i see her videos it makes my day better, i love her sm, she deserves the whole world <3