Yo

'Yo' (よっ) is the most condensed form of casual male greeting in Japanese. It evolved from 'you' (よう, meaning 'hey'), shortened to a single syllable. Typically accompanied by a brief hand raise. It's a classic masculine colloquialism that has existed since before the 1990s, and carries a somewhat older-guy image. Younger men tend to prefer 'ossu' (おっす) or 'uissu' (ういーす). It can be followed by more words like 'yo, hisashiburi' (hey, long time no see) or 'yo, genki?' (hey, how's it going?). Think of it as the Japanese equivalent of 'sup.'