Green’s Dictionary of Slang

daylights n.

1. [mid-18C+] the eyes.

2. [mid-19C+] the insides; the essence; usu. in combs. involving fear and/or violence, see phrs. below.

In phrases

beat the (living) daylight(s) out of (v.) (also bash..., belt..., kick..., knock..., shake..., thrash..., wallop, whale)

[mid-18C+] to beat severely.

darken someone’s daylights (v.) (also ...skylights, dark)

1. [mid-18C–1900s] to black someone’s eye.

2. [late 18C] used as an oath.

frighten the (living) daylights out of (v.) (also scare...)

[late 19C+] to terrify.

send the daylights out of (v.)

[20C+] (W.I.) to beat severely, to hit an animal hard enough to kill it.