To hold in one's stomach when shirtless to appear less fat than you actually are, just as David Hasselhoff does on _Baywatch_. Example: Look at that guy, trying to look in good shape but he's just Hasselhoffing. OR Look at the Hasselhoff on that guy.
HASSELHOFF
Meaning of HASSELHOFF in English
Slang English vocab. Английский сленговый словарь. 2012