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1.
when you stroke your penis up and down it makes an umm - stiss sound
he was beating his penis in the bathroom and you could hear an umm stiss sound from the next room
by Anonymous March 30, 2003
 
2.
What's this?
"Fred drunkenly pulled down the transsexual's skirt, mumbling, 'umm stiss?'"
by Professor Plum July 08, 2003