Can any Definition Work?

So - is epicaricacy a word, or not?  "If  it's not in a current well-respected dictionary," I am told, "it isn't a word.  "It is not used enough."  "The same people publish the word so it's not in the general vernacular."  Blah, blah, blah.

That's all well and good, though, it  has been in a well-respected dictionary in the past.  However, you can't have it both ways.  It must be that way for all words and their definitions then.  For example, we are talking about the French word couac on Wordcraft.  The meaning in French is "a bad note from a defective or mishandled reed instrument."  Some there think that since that meaning is mentioned elsewhere (not in lexicographic instances) that it is a legitimate definition in English, too.  If it's used that way by some, fine, I agree.  But then epicaricacy is an English word as well.  A tit for a tat, so to speak.