Most people don't say Coward + Boy. But you can. It just seems a little awkward.
When you call someone a "Coward" most people Stack Insults. Like if the are a Coward and a Fool (Uninformed and Stupid) Or and "Coward" might only get their strength when they are hiding behind the uniform of a cop which has the protection of violent government.
Thank you Clive. I have one more question if you could kindly answer. There are so many phrasal verbs in the English language. Is there any method to learn them fast or does one need to cram them ?