The itchy, annoying, smelly, totally unnecessary sweat that builds up in your asscrack, just above your asshole (where you'd grow a tail if you had one) - usually on hot days, or after strenuous physical exercise - or all the time on fat people.
I wish they'd come up with an anti-perspirant specifically for crack sweat!
(n.) The sweat that forms in the asscrack after a sustained period of strenuous physical activity, like running in the summer with three pairs of sweatpants and three layers of coats on in Taiwan. Also can be the sweat that forms in your butthole due to an intense masturbation session or from being horny for an excessive amount of time.
After playing an intense game of squash, my shorts were drenched in crack sweat. Most unpleasant.
Michelle made me so horny and hot inside that crack sweat started to form in my bunkhole.