Old English hraþor "more quickly, earlier, sooner," also "more readily," comparative of hraþe, hræþe "quickly, hastily, promptly, readily, immediately," which is related to hræð "quick, nimble, prompt, ready," from Proto-Germanic *khratha- (source also of Old Norse hraðr, Old High German hrad), from PIE *kret- "to shake." The base form rathe was obsolete by 18c. except in poetry (Tennyson); superlative rathest fell from use by 17c. Meaning "more willingly" is recorded from c. 1300; sense of "more truly" is attested from late 14c.
The rather lambes bene starved with cold
[Spenser, "The Shepheardes Calender" (Februarie), 1579]