From XIV onwards -ish was added to adjs. with the sense ‘approaching the quality of, somewhat, rather’, first to adjs. of colour, as blueish, greenish, reddish, but later to any (esp. monosyllabic) adjs., as softish. This use has been extended in XX to the qualification of hours of the day or numbers of years, as four-ish, 1940-ish. Endings of other origin have been assim. to -ish in, e.g., lavish, squeamish.
"-ish." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (September 18, 2018). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/ish-0
"-ish." The Concise Oxford Dictionary of English Etymology. . Retrieved September 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/ish-0