Green’s Dictionary of Slang

1. (US prison) an electric light.

1934		J.L. Kuethe ‘Prison Parlance’ in AS IX:1 26: dimmer. An electric light.
1950		Goldin et al. DAUL 58/2: Dimmer. An electric light; a flashlight. ‘Choke (put out) that dimmer, Slim, this window’s on the stem (facing the street).’.

2. (US) in pl., the eyes.

1954

Hepster’s Dict. 3: Dimmers – Eyes.

3. (UK juv.) a stupid person.

2006

D. Mitchell Black Swan Green 140: Even Dimmest Dimmers Can Succeed If They Never Give Up.