1. Incredibly timid and a massive coward. Emphasis on the superlatives in the previous sentence; this phrase is the pinnacle of all derogatory remarks regarding a person's cowardice.
1. to be a craven cur: This could replace any insult that would otherwise be considered too obnoxious or inappropriate at the time being. If your friend buys milk instead of orange juice, instead of yelling "you fucking idiot!" at him, you could say "craven cur!"
John: What have you done!
Jane: Well, I snuck into your house, ambushed your family, and didn't even give them a chance to fight back.
John: Craven cur!
John: I brought milk home!
Jane: You craven cur! I told you to bring orange juice!