Is there a term for an unrelated word that has evolved to describe something else? EX: saying "pigs" to describe "police officers"

Is there a term for a word or expression that is used as a substitute for something else? Ex: using guns in place of biceps or pigs instead of police officers.

I thought it was a Metonym, but the definition is "a word, name, or expression used as a substitute for something else with which it is closely associated". A metonym uses a closely associated word, but I am looking for one that is not necessarily closely associated.

Solution 1:

It's a metaphor. It's making a comparison from one item to another in some related manner even if that comparison isn't readily apparent in the statement itself.

In order for someone to understand the comparison, you'd have to introduce it as a metaphor at some point. Even if it's physically pointing to that object and saying your new term.

Police officers being called pigs is an insult based upon the characteristics of pigs. (They're dirty, greedy, etc.)

Calling your biceps guns is substituting one manly sounding thing for another.

I agree that metonym isn't a good fit because they're not inherently linked.