This definition is from urbandictionary.com "An insult referring to either the stupidity/low intelligence of another, or the uselessness of an idea, remark, or person"

Did you mean to ask "... a metaphorical [adj.] meaning [noun]" ? Of course you did :) Merrielle is right-on with the Urban Dictionary reference. Though I can't say I have heard this particular phrase before, I am somewhat out-of-the-loop when it comes to modern pop culture language. (I assume your question stems from the 2011 movie title.) However, I can suggest a couple of examples of how it might be used if you want to work it into your own lexicon: ~ "Did you hear that guy? He is dumber than a bag of hammers!" ~ "You think SHE will help you with your research project? She will be about as useful as a bag of hammers."