prefix of reversal, deprivation, or removal (as in unhand, undo, unbutton), Old English on-, un-, from Proto-Germanic *andi- (source also of Old Saxon ant-, Old Norse and-, Dutch ont-, Old High German ant-, German ent-, Gothic and- "against"), from PIE *anti "facing opposite, near, in front of, before, against" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before").
More or less confused with un- (1) through similarity in the notions of "negation" and "reversal;" an adjective such as unlocked might represent "not locked" (un- (1)) or the past tense of unlock (un- (2)).
Old English eorþe "ground, soil, dirt, dry land; country, district," also used (along with middangeard) for "the (material) world, the abode of man" (as opposed to the heavens or the underworld), from Proto-Germanic *ertho (source also of Old Frisian erthe "earth," Old Saxon ertha, Old Norse jörð, Middle Dutch eerde, Dutch aarde, Old High German erda, German Erde, Gothic airþa), perhaps from an extended form of PIE root *er- (2) "earth, ground."
The earth considered as a planet was so called from c. 1400. Use in old chemistry is from 1728. Earth-mover "large digging machine" is from 1940.