"lump of earth or clay," Old English clod- (in clodhamer "the fieldfare," a kind of thrush, literally "field-goer"), from Proto-Germanic *kludda-, from PIE *gleu- (see clay).
Synonymous with collateral clot until meaning differentiated 18c. Meaning "person" ("mere lump of earth") is from 1590s; that of "blockhead" is from c.1600 (cf. clodpate, clodpoll, etc.). It also was a verb in Middle English, meaning both "to coagulate, form into clods" and "to break up clods after plowing."