"of the nature of a clod," hence "base, low, boorish," 1838, from clod (n.) + -ish. Related: Clodishly; clodishness.
Entries linking to cloddish
clod n.
"lump of earth or clay," Old English clod- (in clodhamer "the fieldfare," a kind of thrush), from Proto-Germanic *kludda-, from PIE *gleu- (see clay).
Synonymous with collateral clot until the meanings differentiated 18c. Meaning "person" ("mere lump of earth") is from 1590s; that of "blockhead, dolt, stupid fellow" is from c. 1600 (compare clodpate, clodpoll, etc. in the same sense). It also was a verb in Middle English, meaning both "to coagulate, form into clods" and "to break up clods after plowing."
-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.