Green’s Dictionary of Slang

1. (UK tramp) to get sunburnt.

1933

M. Marshall Tramp-Royal on the Toby 52: When we lie flat on our back we feel only the sun, then, when we sit up, only the wind [...] we will be kippered in no time.

2. to get singed by a fire.

1966

G. Fletcher Down Among the Meths Men 78: I waited in the rain, getting as near to the fire as I could without being kippered.