"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."