Armpit fat. The small roll of fat between the armpit and side of the breast. Used in a similar vain to muffin top
(referring to waist fat above the the hip bone). Your pit puppies are not legitimate side-boob, but are a separate roll of fat that leaks out of the side of the bra and ends at the armpit crease, although a pit puppy is sometimes only visible when wearing a bra.
"I can't wear that tank top because my pit puppies come out to play"
"Ouch my bag strap just pinched my pit puppy"
"I've put on a fresh layer of fat, check out my pit puppies, I gotta get back to the gym"
"It doesn't matter how much I work out, I just can't get rid of these damn pit puppies"
Guy:"That girl's breasts are leaking out the side of her bra"
Girl: "Nah, that is not breast, it's just pit puppies"