Green’s Dictionary of Slang

ass-buster n.

[ass n. (2) + buster n.6 ]

1. [1950s] (US) a dive in which a swimmer jumps, holds their nose and hits the water buttocks-first, the aim, and the result, being to make a big splash.

2. [1970s] an outstanding person.

3. [2000s] a fall.

4. [2000s] something challenging.