I was reading Captain America and The First Thirteen and Cap said "alley-oop" as he hopped out of his window. It got me thinking of where this phrase came from. Cap was in France during World War II so I thought maybe the phrase should actually be "allez-oop?" Aller is the French word for to go. The tenth Doctor Who is famous for saying "Allons y!" meaning "let's go!"
I checked online and found a couple different definitions. According to wordsmith.com
interjection: Used as an exhortation or to signal the start of an activity. For example, when coordinating efforts to lift something heavy.
noun: A basketball move in which a player throws the ball to a teammate near the basket who leaps to catch it in mid-air and then puts it in the basket before returning to the floor.
The Oxford English Dictionary adds that alley-oop is a maneuver used in skateboarding or snowboarding spin usually involving an 180 degree or more turn.