Old English full "completely, full, perfect, entire, utter," from Proto-Germanic *fullaz (cf. Old Saxon full, Old Frisian ful, Old Norse fullr, Old High German fol, German voll, Gothic fulls), from PIE *pele- (1) "to fill" (see poly-).
Adverbial sense was common in Middle English (full well, full many, etc.). Related: Fuller; fullest. Full moon was Old English fulles monan; first record of full-blood in relation to racial purity is from 1812.