What is the X in X-mas?
"one who fulls cloth," Old English fullere, from Latin fullo "fuller" (see foil (v.)). The substance called fuller's earth (silicate of alumina) is first recorded 1520s, so called because it was used in cleansing cloth.
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. Full house is 1710 in the theatrical sense, 1887 in the poker sense.
"to tread or beat cloth to cleanse or thicken it," late 14c., from Old French fouler, from Latin fullo (see foil (v.)); Old English had the agent-noun fullere, probably directly from Latin fullo.
The word "full" is from the Anglo-Saxon fullian, meaning "to whiten." To full is to press or scour cloth in a mill. This art is one of great antiquity. Mention is made of "fuller's soap" (Mal. 3:2), and of "the fuller's field" (2 Kings 18:17). At his transfiguration our Lord's rainment is said to have been white "so as no fuller on earth could white them" (Mark 9:3). En-rogel (q.v.), meaning literally "foot-fountain," has been interpreted as the "fuller's fountain," because there the fullers trod the cloth with their feet.