Ah! Well, I can give you a guess about the answer. In casual speak, if it ends in -んだ or -の, the last syllable of the preceding word - particularly -る - is slurred into ん.

So - 冷やってんだ might be 冷やってるんだ - or, if you want grammatically correct, 冷やっているんだ.

That would be my guess, at least. You sure it was a command, though? Because I don't think that particular slurring would happen with an んだ imperative form. I've always understood (X in plain form) + んだ to be "You will do X." A command, but it's a command because you're stating it as absolute fact...

...Feel free to tell me to shut up, by the way. I love words and I love comparing how words work across languages...