late Old English ridere "one who rides, trooper, knight, mounted warrior," agent noun from ride (v.). Compare Middle Dutch rider, Dutch rijder, Old High German ritari, German Reiter, Old Norse riðeri; "It is uncertain how far the forms were independently developed in these languages" [OED]. As "one who is carried along on or in" (a chariot, cart, etc.), from c. 1200. Meaning "clause tacked on to a document after first draft" is from 1660s.
word-forming element meaning "lacking, cannot be, does not," from Old English -leas, from leas "free (from), devoid (of), false, feigned," from Proto-Germanic *lausaz (cognates: Dutch -loos, German -los "-less," Old Norse lauss "loose, free, vacant, dissolute," Middle Dutch los, German los "loose, free," Gothic laus "empty, vain"), from PIE root *leu- "to loosen, divide, cut apart." Related to loose and lease.