Why is the ninth month called September?
Old English stocc "stump, post, stake, tree trunk, log," also "pillory" (usually plural, stocks), from Proto-Germanic *stukkaz "tree trunk" (cf. Old Norse stokkr "block of wood, trunk of a tree," Old Saxon, Old Frisian stok, Middle Dutch stoc "tree trunk, stump," Dutch stok "stick, cane," Old High German stoc "tree trunk, stick," German Stock "stick, cane;" also Dutch stuk, German Stück "piece"), from PIE *(s)teu- (see steep (adj.)).
Meaning "ancestry, family" (late 14c.) is a figurative use of the "tree trunk" sense (cf. family tree). This is also the root of the meaning "heavy part of a tool," and "part of a rifle held against the shoulder" (1540s). Stock, lock, and barrel "the whole of a thing" is recorded from 1817. Meaning "framework on which a boat was constructed" (early 15c.) led to figurative phrase on stocks "planned and commenced" (1660s). Stock-still (late 15c.) is literally "as still as a tree trunk."
"supply for future use" (early 15c.), "sum of money" (mid-15c.), Middle English developments of stock (n.1), but the ultimate sense connection is uncertain. Perhaps the notion is of the "trunk" from which gains are an outgrowth, or obsolete sense of "money-box" (c.1400). Meaning "subscribed capital of a corporation" is from 1610s.
Stock exchange is attested from 1773. In stock "in the possession of a trader" is from 1610s. Meaning "broth made by boiling meat or vegetables" is from 1764. Theatrical use, in reference to a company regularly acting together at a given theater, is attested from 1761. Taking stock "making an inventory" is attested from 1736. As the collective term for the movable property of a farm, it is recorded from 1510s; hence livestock.
"to supply (a store) with stock," 1620s, from stock (n.2). Related: Stocked; stocking.
in reference to conversation or literature, "recurring, commonplace" (e.g. stock phrase), 1738, from stock (n.2) on notion of "kept in store for constant use."