A Korean term that literally means something like "Uncle", and is a polite way to refer to a man who is older than you, who you don't know well. That doesn't mean they're _old_, necessarily-- little kids might call a man in his 20's "Ajusshi".
It could also be used as the equivalent of English "Sir" or "Mister", when speaking to a stranger.
Let me help you across the street, Ajusshi.
Hey Ajusshi, do you have the time?