mid-12c., "flag or other conspicuous object to serve as a rallying point for a military force," from shortened form of Old French estandart "military standard, banner." According to Barnhart, Watkins and others, this is probably from Frankish *standhard, literally "stand fast or firm," a compound of unrecorded Frankish words cognate stand (v.) and hard (adj.). So called because the flag was fixed to a pole or spear and stuck in the ground to stand upright. The other theory [OED, etc.] calls this folk-etymology and connects the Old French word to estendre "to stretch out," from Latin extendere (see extend). Some senses (such as "upright pole," mid-15c.) seem to be influenced by if not from stand (v.). Standard-bearer in the figurative sense is from 1560s.
1620s, "serving as a standard," from standard (adj.). Earlier it meant "upright" (1530s). Standard-bred "bred up to some agreed-upon standard of excellence" is from 1888.
"weight, measure, or instrument by which the accuracy of others is determined," late 14c., from standard (n.1) "military standard, banner," a particular use in English of this word, but the sense evolution is "somewhat obscure" [OED]. The standard weights and measures were set by royal ordinance and were known as the king's standard, so perhaps metaphoric, the royal standard coming to stand for royal authority in matters like setting weights and measures. Hence the meaning "authoritative or recognized exemplar of quality or correctness" (late 15c.). Meaning "rule, principal or means of judgment" is from 1560s. That of "definite level of attainment" is attested from 1711 (as in standard of living, 1903).