An Armpit fuck is a rare sexual activity between males and mostly females. He sticks his erect penis from behind between her chest and upper arm, in her armpit (preferably sweaty for lubrication) and starts moving his penis forth and back, thus fucking her armpit. Usually this activity is accompanied by him stroking his dick or her helping by adding handjob strokes or even short blowjobs. Men who enjoy to fuck armpit are quite often armpit voyeurs. To fuck an armpit is also called auxiliary intercourse. The aftermath of fucking her armpit is dripping cum from her armpit.
If not sweaty, Lubrication is highly recommended.