What is meaning of fertilizer as bad word?

Answer 1

Fertilizer itself is not a bad word and is not used as a bad word in conversation. But because it is often made from horse manure or cow manure, it can be used as a "euphemism" (a polite way of saying something vulgar). "Horse manure!" is the polite way of saying something is total nonsense, completely false; a slang word for this is "crap" (or "rubbish" in British slang).