In Legends of Tomorrow, Vandal Savage called Rip Hunter by the name
Gareeb, even when he meets him for the first time in the Egyptian era.
Why did he nickname him like this? What does it mean? Is it some reference?
As was shown or alluded to in the show, Hunter tries to travel back in time to kill Vandal in ancient Egypt, before he obtained his immortality. Gareeb is evidently Arabic for, "the stranger", so it's likely in reference to Vandal not knowing who Hunter was at that time as he was quite literally a stranger who attempted to kill him.
It's actually not Arabic, it's Urdu and in Urdu it means "Poor." http://hamariweb.com/dictionaries/poor_urdu-meanings.aspx
The creators just picked a word that meant "stranger" or "foreigner without thinking that it's an Arabic word, not an Ancient Egyptian one. Then again, we have no idea how Ancient Egyptian sounded, so they probably went with what sounded cool.