"胸クソ" is "Muna kuso". "Muna" means the breast, "kuso" means s*hit.
"Muna kuso" means "chest's ****"? :?
In this case, "Kuso" has not important meaning. "Muna kuso" is only a slang of "Mune". And "Mune(Munakuso) ga warukunaru" means "unpleasant" or "disagreeable".
The Beyond Polite Japanese dictionary has it as slightly stronger "sickening, disgusting, pukey, getting all bummed out." Sounds like from the same place the English expression feeling like ****, feeling ****** came from.