Green’s Dictionary of Slang

a body louse.

1920		Capt. Billy’s Whiz Bang Oct. : There once was a girlie from Litchen, / Stood scratching herself in the kitchen, / Her father said, ‘Rose, / Coots, I suppose’.
1928		(con. 1918) L. Nason Top Kick 16: It’s about time or the coots would eat us alive. The place is lousy with them.
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.