Green’s Dictionary of Slang

lights n.

[SE lights, offal, often considered an inedible piece of the animal]

a fool.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 169: lights a ‘cake,’ a fool, a soft or ‘doughy’ person.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.