Anything that is extremely tiring and thoroughly physically taxing which results in aches and pains. Such as 2 hours of muay thai, a full day of snowboarding, or single handedly fighting a contingent of angry midgets.
A: Jesus, you look like shit.
B: Ugh. yeah. just had a full body fuck down.
B: 6 hours of advanced secret ninja assassin training. at 3am.
A: oh damn.