late 15c., "fountain, stream," of uncertain origin, probably from French sourge-, stem of sourdre "to rise, swell," from Latin surgere "to rise, arise, get up, mount up, ascend; attack," contraction of surrigere, from assimilated form of sub "up from below" (see sub-) + regere "to keep straight, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule"). Meaning "high, rolling swell of water" is from 1520s; figurative sense of "excited rising up" (as of feelings) is from 1510s.
late 13c., sustenen, transitive, "provide the necessities of life to;" by early 14c. as "give support to; support physically, hold up or upright; give assistance to; keep (a quarrel, etc.) going," from the stem of Old French sostenir, sustenir "hold up, bear; suffer, endure" (13c.), from Latin sustinere "hold up, hold upright; furnish with means of support; bear, undergo, endure." This is from an assimilated form of sub "up from below" (see sub-) + tenere "to hold" (from PIE root *ten- "to stretch").
The meaning "continue, keep up" (an action, etc.) is from early 14c. The sense of "endure (pain hardship, a shock) without failing or yielding" is from c. 1400. The legal sense of "admit as correct and valid" is from early 15c. Past-participle adjective sustained is by 1775 as "kept up or maintained uniformly," originally of music notes; the piano's sustaining pedal is so called by 1889.
1590s, "a swallowing up" (now obsolete), from Latin absorptionem (nominative absorptio) "a swallowing," noun of action from past-participle stem of absorbere "swallow up" (see absorb). From 1714 specifically of "disappearance by assimilation into something else."