c. 1200, ridden, "clear (a space); set free, save," from Old English *ryddan (past participle geryd) or else from a Scandinavian source akin to Old Norse ryðja (past tense ruddi, past participle ruddr) "to clear (land) of obstructions," from Proto-Germanic *reudijan (source also of Old High German riuten, German reuten "to clear land," Old Frisian rothia "to clear," Old English -royd "clearing," common in northern place names), from PIE root *reudh- "to clear land."
Meaning "be rid of, be freed from" (something troublesome or useless) is from mid-15c. The general sense of "to make (someone or someplace) free (of someone or something else)" emerged by 16c. The senses have merged somewhat with those in Northern English, Scottish, and U.S. dialectal redd (q.v.). To get rid of (something or someone) is from 1660s. Related: Ridden; ridding.