So, apparently "trap" is a slang for mouth (according to freeonlinedictionary anyway). The book I read, describes a man with a "fierce trap of a mouth". If "trap" is slang for mouth, it basically translates as a man with a "fierce mouth of a mouth", which doesn't make any sense... whatsoever!

Anyone?

The context, in case there's any doubt:

"The leader of the guards, Priam was a lanky man with a fierce trap of a mouth."