Well, when Rebels fight Imperials, they are predominantly facing two types of foes:
- TIE Fighters
Stormtroopers are frequently called "bucketheads", a term that originated from the Republican period when clone armor were considered as little better than serving as body buckets - only good for keeping your body in one piece. In the Imperial period, it became a derogatory term as the Empire starts making everyone's lives miserable. Sure, everyone unhappy with the Empire uses it, but as long as Rebels also use it, that counts right?
Notable users (according to Wookieepedia's article on the term):
- Ezra Bridger: once to Stormtroopers guarding his detention cell, second to Agent Kallus (who wears a similar bucket)
- Han Solo: in reference to TIE pilots
- Poe Dameron: in reference to First Order Stormtroopers
I'm sure Rebel pilots have a word for TIE pilots, who literally come in annoying swarms but drop like flies (TIE Fighters are notorious easy to 1-shot), but I can't recall any offhand just yet...