When a male moves rapidly i.e. jogs, runs, or gallops at a fast rate and their testicles and/or penis hits their inner thigh area (unilaterally or bilaterally, depending on which way you lean and how fast you're moving) creating a 'SLAP' noise that can be heard by others.

Cause:
Commonly by not wearing any supporting undergarments (i.e. underpants) which allows the gonads to move free-flowingly.

Action:
Wear underpants or some other type of supporting undergarment (stupid)

WARNING!! Can be painful!

N.B. Through recent study it has been noted that women can also suffer from a female equivalent of 'chap slap'.
However, more studies have to be conducted in order to confirm the origins of the 'SLAP' noise.
Scientists are unsure whether it originates from the genitals, buttocks thighs or overtly large and gravitationally affected mammary tissue(s).

Biologically female candidates will be required for a forthcoming study - Sadly, due to poor sponsorship, candidates will not be rewarded for their efforts.

A casual invitation / threat used by an individual that is not afraid of fucking someone up.

When a male gets slapped in the gonads by another male and the latter shouts "CHAP SLAP!"