YOSI is a slang derivative of the Tagalog word SIgarilYO (from Spanish, cigarrillo for cigarette), with its first and last syllables put together in reversed order.
cigarrillo = sigarilyo = SI garil YO = SIYO = YOSI
> Huy! Mag-yosi muna tayo...
> "Hey! Let's have a cigarette first..."