"portion," Old English scearu "a cutting, shearing, tonsure; a part or division," related to sceran "to cut," from Proto-Germanic *skeraz (source also of Old High German scara "troop, share of forced labor," German Schar "troop, band," properly "a part of an army," Old Norse skör "rim"), from PIE root *sker- (1) "to cut."
Meaning "part of the capital of a joint stock company" is first attested c. 1600. Share and share alike attested from 1560s. The same Old English noun in the sense "division" led to an obsolete noun share "fork ('division') of the body at the groin; pubic region" (late Old English and Middle English); hence share-bone "pubis" (early 15c.).
"iron blade of a plow," Old English scear, scær "plowshare," properly "that which cuts," from Proto-Germanic *skeraz (source also of Old Frisian skere, Middle Low German schar, Old High German scar, German Schar, Dutch ploegschaar, Middle High German pfluocschar), from PIE root *sker- (1) "to cut."
1580s, "to apportion to someone as his share; to apportion out to others; to enjoy or suffer (something) with others," from share (n.1). Meaning "to divide one's own and give part to others" is recorded from 1590s. Meaning "confess one's sins openly" (1932, implied in sharing) is from "the language of Moral Rearmament" [OED]. Related: Shared; sharer; sharing.