Trading blows

Trading blows or trading licks is an endurance test in which the participants take turns, alternating between administering a blow to an opponent and assuming the agreed exposed position to endure the next one, using the same implement, until only the winner can still bring himself to endure the gradually increasing pain in the progressively tormented target part of their anatomy, which in the interest of fairness should be covered by a common uniform. This can be anything from regular jeans or pants, underwear, and finally, bared buttocks. As the blows are not given by the same person but by the parties themselves, the strongest-armed one actually has an unfair physical advantage.
Such rather macho displays of willpower, restraining the instinct to avoid pain, can serve various purposes, including:

a physical punishment, especially for quarreling, possibly the origin of the practice
a motivation test, especially as part of an initiation process, such as hazing
an obedience test, as in certain paddle games
a duel, either personal or as champions representing similar, especially rivaling, groups; in certain fraternities, refusing such a challenge may result in exclusion from the membership, even for an alumnus
as a game, either to 'proudly' display one's tenacity or in the pursuit of a sadist and/or masochistic, erotic or pain-addicted, kick.
as a sexual fetish

Another game with the same name is often played among boys or young men, where two people agree on a place to hit the other and the two take turns trading punches until one person cannot stand the pain any longer. The first person to give up is the loser.