"slightly, to a small extent," 1835, from rather + -ish.
Entries linking to ratherish
rather adv.
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)."
-ish
adjectival word-forming element, Old English -isc "of the nativity or country of," in later use "of the nature or character of," from Proto-Germanic suffix *-iska- (cognates: Old Saxon -isk, Old Frisian -sk, Old Norse -iskr, Swedish and Danish -sk, Dutch -sch, Old High German -isc, German -isch, Gothic -isks), cognate with Greek diminutive suffix -iskos. In its oldest forms with altered stem vowel (French, Welsh). The Germanic suffix was borrowed into Italian and Spanish (-esco) and French (-esque). Colloquially attached to hours to denote approximation, 1916.
The -ish in verbs (abolish, establish, finish, punish, etc.) is a mere terminal relic from the Old French present participle.