Green’s Dictionary of Slang

1. (US) an individual, obect or situation seen as being of lasting value; thus a person who may provide a lasting relationship.

1957		‘Weldon Hill’ Onionhead (1958) 133: She isn’t a keeper, I’ll throw her back.
1981		D. Jenkins Baja Oklahoma 7: The sky was bluer than a bathroom wall. As the days came and went for Juanita Hutchins, she called it a keeper.
1995		G. Pelecanos Down by the River 11: ‘[T]hat woman I got now, that church woman? She’s a keeper’.
2000		W.T. Vollmann Royal Family 540: I hope you mean to take good care of her. She’s a keeper.
2007		R. Milward Apples (2023) 15: [We] gazed at him buying us vodka and cokes. He was a keeper.
2008		D.R. Pollock ‘Rainy Sunday’ in Knockemstiff 142: ‘Oh, don’t talk about my new man like that. He might end up being a keeper’.
2019		D. Jenkins Reunion 101: [T]he town offered its residents precisely 273 saloons, dance halls, gambling dens, and brothels as opposed to one church. As stats go, I call that a keeper.

2. any form of weapon [i.e. it keeps one safe].

1992

Bentley & Corbett Prison Sl. 87: Keeper Any weapons.