The phrase comes from everyday Mexican Spanish, where carnal is a warm way to address a close friend and la costumbre means the custom or the habit. Online the short recording circulates as a standalone reaction clip rather than a longer song or scene, shared in chats and short edits when a familiar pattern shows up again. Its appeal is brevity and tone, a two second line that sums up you always do this without needing explanation, which keeps it useful for jokes between friends who already know the routine well.