Third Country National, the official US military term for citizens of nations other than the US and the local country. Mostly employees of KBR and the other big contractors, most TCNs are Indian, Pakistani, Bangladeshi, or Filipino, and do the majority of the menial labor on US bases throughout Iraq and Afghanistan. The specific title exists to make the important distinction between these contractors (who are indirectly employed by the US government) and direct government employees on the one hand, and local nationals on the other.
"Do those TCNs have any job besides cleaning our bathroom? They're always in there!"
"Seriously, I think they sleep in there. Saves KBR the expense of giving them a room."