«مَا تِسْتَكْتَرْشِ الرَّفْصْ عَلَى الْبَغْلِ النِّجِسْ»

Here نجس describes not an impure animal but a cunning mule prone to bolting, one that can throw its rider and even kill him, so a mere kick (الرفص, colloquial for رفس) is the gentlest trouble it can cause. The proverb asks the listener not to treat that small offense as excessive or unexpected.

Applied to people, the saying marks someone as crafty and ill-natured and warns that no misdeed from them should seem unlikely. A minor wrong from such a person is no surprise. Real caution should go toward the greater harm they are capable of.

Use it when someone known for slyness or bad conduct does something mildly harmful and a listener seems shocked. It says the act is minor beside what that person could do. Critical in tone, it targets the offender's character.