Definition of "napper"

You look so dapper from your napper to your feet.

1911, Charles Collins, Fred E. Terry, E.A. Sheppard (lyrics and music), “Any Old Iron”

Under fire when this magazine is emptied you shout for “ammo” but perhaps No. 2, the ammo carrier, is lying in the rear with a bullet through his napper.

1917, Arthur Guy Empey, “Lewis Gun”, in Over the Top, G. P. Putnam's, page 297

As he walked back to his flat after meekly apologising, he wondered why a severe pain in his napper could affect the lead in his pencil.

2011, Jeremy Roberts, Let's Not And Say We Did, page 96