Literally: big – liver/gallbladder / courage – non – enemy
Alternately: Being, or at least acting, completely unafraid. Acting as confident as someone without any (meaningful) enemies.
Notes: This is another compound comprising two near-synonyms. 大胆 is “brave(ry),” while 不敵 isn’t so much a lack of antagonists, as it is a state of being bold enough to not even acknowledge or recognize your enemies as enemies.
Although the phrase itself is relatively neutral, it may be used to obliquely criticize those whose courage mostly seems to spring from an overabundance of self-confidence or self-assertion.
Note that this phrase, and several related ones, come from an archaic belief that the liver (which contains the gallbladder) was the seat of not just courage, but of the soul itself, within the human body.