The phrase "armpit fat" refers to the excess fat that can accumulate in the area between your upper arm and chest.
Sentences with «armpit fat»
And as you can see from the dress on the Budget Fashionista's blog, many of them are not kind to women with armpit fat (and that dress is on a model). (thefashionablehousewife.com)
I had a strapless, and I was a bit heavier than I usually like to be, and I didn't have armpit fat in any of my pictures. (itsbecauseithinktoomuch.com)