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.
"the earth regarded as placed midway between heaven and hell or the abode of the gods and the underworld," late 13c., from middle (adj.) + earth. Altered from earlier middel-erd (late 12c.), midden-erd, itself an alteration (by association with Middle English eard "dwelling") of Old English middangeard (see Midgard).