The story
From 360, the number of degrees in a full rotation, + backflip, the class of maneuvers that begin similarly.
Etymology from Wiktionary, CC BY-SA 4.0.
360 backflip
Today it means: A maneuver in various sports (such as skateboarding) in which the player flips backwards, rotates a full turn, and finishes travelling in the original direction. Full definition →