Bullshit, nonsense, something that is a lie. However, crock is not usually used as an interjection like the above words may be.
Crock is quickly being picked up as a retort for proverbs or phrases such as; "You can do anything you set your mind to," or, "Don't put all your eggs in one basket.
Crock can also be used to describe a phony or false thing or someone who is a sham.
"Two wrongs don't make a right?" What a crock! Two lefts don't make a right either jackass!
"Give a man a fish, and he'll eat for a day. Teach a man to fish, and he'll eat forever." Unless of course, the man gets tired of eating fish, because no human can eat the same thing for their entire life without getting pissed off at it. And unless, the fish is poisoned, not cleaned properly before eaten, etc. Definitely a crock.
"Fine feathers make fine birds." Right, except for the part where birds aren't made out of feathers, idiot.
Remember that the word "crock" isn't limited to insulting phrases, though.