A sexual act where a male repeatedly slides his penis under a female's (or male's) armpit as an alternative to penis to vag intercourse*.
[Armpit sex] may also be referred to as [armpit fucking], pit fucking, putting one's dick in a [headlock], or the pocket of paradise (referring to the armpit).
*Disclaimer: Be careful when experimenting with [armpit sex] as it may result in rashes or serious chaffing.