Much like Underboob...

Bottom tit cleavage... (.)(.)

Also... under tatas... and the thousands of others....

When a woman (or man) has gained sufficient weight such that underneath their breasts (or manboobs) are two folds of fat forming a smaller secondary cleavage, it can be said that they have developed "undertits". Double undertits may be sighted in rarer cases, with triple undertits being an extreme rarity, while quadruple undertits is the oft-rumored holy grail.