the fat that squeezes out of a strapless top where the chest meets the armpit.

A pubic hair piece. A toupee for the pubic area/genitals. In the 1700’s when mercury was used to treat sexually transmitted diseases (Gonorrhea or Syphilis) one of the side effects was the loss of pubic hair. To disguise this condition, that was not cured by mercury, a Merkin was employed.

Where a woman is so fat her tits bulge out of her armpit area causing arm tits

a breast that sags and hides in a woman's armpit when she is laying horozontal.

Flab in the armpit that is getting out of hand

the roll of fat that appears when you put your arm down near your armpit