Green’s Dictionary of Slang

1. a bullet.

1889

B. Mitford Fire Trumpet II 221: Crack! Another report, and the fierce warrior falls, this time stone-dead, leaping nearly against the barrel which at point-blank had sent a full charge of ‘loopers’ straight through his heart.

2. a punch, a blow.

1914

E. Pugh Cockney At Home 203: If you pipe me off like that there agin I’ll give you sich a bloomin’ ’alf-looper you’ll think it’s your bloomin’ birthday!