"somewhat thin, not very stout," by 1830, from slim (adj.) + -ish.
Entries linking to slimmish
slim adj.
1650s, "thin, slight, slender," usually with suggestion of gracefulness, from Dutch slim "bad, sly, clever," from Middle Dutch slim "slanting, crooked; bad, wrong," from Proto-Germanic *slembaz "oblique, crooked" (source also of Middle High German slimp "slanting, awry," German schlimm "bad, cunning, unwell"), which is of unknown origin. Italian sghembo "crooked, slanting, lopsided" is from Germanic.
Not found in Middle English. The Germanic sense evolution seems to be "slanting" to "slight, insignificant" then "gracefully slender." Down another path, "slanting" to "crooked" to "bad, sick, wrong."
Of chances, etc., "meager, small" from 1670s. The sense of "slight, flimsy, unsubstantial" is by 1813, of fabric. In English 17c. also sometimes in reference to persons with a sense "sly, cunning, crafty." Related: Slimly; slimness.
With obsolete extended adjectival forms slimsy "flimsy, unsubstantial" (1845, American English, of fabric, etc.); slimikin "small and slender" (1745). Slim Jim attested from 1887 in sense of "very thin person;" from 1902 as a type of slender cigar; from 1975 as a brand of meat snack. Slim volume "book of verse by a little-known or aspiring poet" is by 1920.
-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.