1. An expression of indifference, apathy, or nonchalance. "Feck" here is a euphemistic, though still mildly vulgar, substitution for "fuck." Primarily heard in Ireland. You know what, feck it, I'm tired of waiting; let's go home.
2. An exclamation of dismay, exasperation, anger, or frustration. Primarily heard in Ireland. Ah, feck it! I can't find my keys anywhere! Feck it, I just had the car fixed and now you've put a dent in it!