Green’s Dictionary of Slang

1. (US) esp. of a horse, to frighten or become frightened.

1926		Will James Smoky 40: Maybe a rider had been spotted that morning which had kettled ’em into a run.
1933		(con. WWI) L. Nason Among the Trumpets 127: Keep your head! Don’t get kettled! Compree? Follow me!

2. (UK prison) add sugar to boiling water in a kettle, then throw it in a victim's face; the victim is scalded and scarred by the sugar which adheres to the skin.

2020

G. Krauze What They Was 262: Berutus tells me about someone getting kettled [...] you boil your kettle, put bare sugar into the boiling water and then dash it in a man’s face.