The literal translation would be milk bag. 乳袋 has three possible meanings.
Chibukuro = Boobs.
Chibukuro (2) = The protruding part of a shamisen or a hand drum.
Chichibukuro = The shape of the boobs clearly visible through clothing.
I guess the first meaning is what is intended and does translate to cowtits. Personifying someone as boobs implies that they have huge ones, after all.
Whoever designed Sento's costume is a prodigy much better than Kanie. Wacky Waki.