Manky means something like dirty or dingy, and the 'combats' refers to combat-style pants. So 'manky old combats' means 'dirty old pants'. :)

I knew you'd pop up. You'll note that I used Britons this time per your subtle suggestion :)

Morgan's answer is correct. So there's at least one 'Yank' here who understands what 'Brits' say.

My thoughts exactly. Maybe a Briton will enlighten us Yanks.

Just wanted to say that as an American English speaker this entire sentence made absolutely no sense to me until I read the answer. I thought I had a stroke. This is very very British English I believe