Old English gadrian, gædrian "unite, agree, assemble; gather, collect, store up" (transitive and intransitive), used of flowers, thoughts, persons; from Proto-Germanic *gaduron "come or bring together, unite" (source also of Old English gæd "fellowship, companionship," gædeling "companion;" Middle Low German gadderen; Old Frisian gaderia; Dutch gaderen "to gather," gade "spouse;" German Gatte "husband;" Gothic gadiliggs), perhaps from PIE *ghedh- "to unite, join" (see good (adj.)). Change of spelling from -d- to -th- is 1500s, reflecting earlier change in pronunciation (as in father). Related: Gathered; gathering.
Old English up, uppe, from Proto-Germanic *upp- "up" (source also of Old Frisian, Old Saxon up "up, upward," Old Norse upp; Danish, Dutch op; Old High German uf, German auf "up"; Gothic iup "up, upward," uf "on, upon, under;" Old High German oba, German ob "over, above, on, upon"), from PIE root *upo "under," also "up from under," hence also "over."
As a preposition, "to a higher place" from c. 1500; also "along, through" (1510s), "toward" (1590s). Often used elliptically for go up, come up, rise up, etc. Up the river "in jail" first recorded 1891, originally in reference to Sing Sing, which is up the Hudson from New York City. To drive someone up the wall (1951) is from the notion of the behavior of lunatics or caged animals. Insulting retort up yours (scil. ass) is attested by late 19c.
1550s, "to drive and catch (swans)," from up (adv.). Intransitive meaning "get up, rise to one's feet" (as in up and leave) is recorded from 1640s. Sense of "to move upward" is recorded from 1737. Meaning "increase" (as in up the price of oil) is attested from 1915. Compare Old English verb uppian "to rise up, swell." Related: Upped; upping. Upping block, used for mounting or dismounting horses, carriages, etc., is attested from 1796 (earlier was horsing-block, 1660s).
