Middle English rather, from Old English hraþor "more quickly; earlier, sooner," also "more readily or willingly," comparative of rathe (Old English hraþe, hræþe) "immediately, quickly, hastily, speedily; promptly, before long; readily," which is related to hræð "quick, nimble, prompt, ready," from Proto-Germanic *khratha- (source also of Old Norse hraðr, Old High German hrad), which is said to be from PIE *kret- "to shake."
The rather lambes bene starved with cold
[Spenser, "The Shepheardes Calender" (Februarie), 1579]
Meaning "on the contrary, in contrast to what just has been said" is from late 13c.; that of "more properly, more truly" is recorded from late 14c. Meaning "preferably" is from c. 1300. Sense of "to some extent, in a greater degree" is from 1590s, that of "somewhat, moderately" is by 1660s.
The adverb rathe was obsolete by 18c. except in poetry (Tennyson); the superlative rathest "earliest, soonest, first" fell from use by 17c. Middle English formed an alternative superlative ratherest (c. 1400) and also had rathely "quickly, swiftly; immediately" (early 14c.) and a noun rather "the former (persons)."