Fusion of Middle English redden ("to save, rescue, deliver, rid, free, clear"), from Old English hreddan ("to save, deliver, recover, rescue"), from Proto-Germanic *hradjaną and Middle English reden ("to clean up, clear"), from Old English ġerǣdan ("to put in order, arrange, prepare"), from Proto-Germanic *garaidijaną ("to arrange"). More at rid, ready.
From Middle English, from Old Norse ryðja, Middle Low German, compare Dutch redden. In modern use probably actually a .
Origin obscure, possibly from the act of the fish scooping, clearing out a spawning place, see redd above.