Ooh, I can help!

*Ahem.

Once upon a time, there was a German state (The real term is rather confusing) called Hesse-Kassel. The ruler needed money, and the Brits needed soldiers (Good relations between the two as well) So they paid for some soldiers from Hesse-Kassel, as well as some other places. The majority were Hessians, so that's why they called them that. Not really mercs, more like diplomatic possesions. But they definitely weren't British troops recruited from British lands.

They weren't "Elite" soldiers, but they were well-trained, and generally did well in battle. That's why Washington defeating them is so famous. The colonials didn't like them very much in the first place, what with not speaking English and all, so it was alright.