Describing damage resistance

[Dualazi]

I've actually found the most sensible description for me is "The damage is halved by the monster's resistance." I don't even skirt around it, I tell them outright.

As you say, the players can picture it for themselves, plus they don't have to wait for me to come up with some clever description, or fumble over a terrible one.

I do this, and then usually play up their toughness/resistance to the attacks as combat progresses as a more narrative-friendly reminder. Initially though I feel there's little value in trying to obfuscate it, players will either end up annoyed by the misinformation or the DM gets exasperated like the OP. I take a similar approach with status effects, if a creature is stunned, I'll simply tell them so.