Axillism is a sexual paraphilia in which one is sexually attracted to the armpit. This is also referred to as maschalagnia.
More About Axillism
This particular fetish is similar to other fetishes that focus on one specific body part. The act of using the armpit as a sexual area is done similarly to a foot job, titty fuck, or a hand job by inserting the penis between the torso and the arm, which is the armpit and moving back and forth. For those who experience axillism, though, the idea of having sex with the armpit is much more exciting and arousing that the prospect of intercourse with a vagina.