late 13c.,sustenen, transitive, "provide the necessities of life to;" by early 14c. as "give support to (an effort or cause); also in physical senses, "keep from falling or sinking, hold up or upright;" also "give assistance to; keep (a quarrel, etc.) going." It is from the stem of Old Frenchsostenir, sustenir "hold up, bear; suffer, endure" (13c.), from Latinsustinere "hold up, hold upright; furnish with means of support; bear, undergo, endure." This is from an assimilated form ofsub "up from below" (seesub-) +tenere "to hold" (from PIE root*ten- "to stretch").

The meaning "continue, keep up" (an action, etc.) is from early 14c. The sense of "withstand, endure (pain hardship, a shock) without failing or yielding" is from c. 1400. The legal sense of "admit as correct and valid, uphold the rightfulness of" is from early 15c. Also from early 15c. as "suffer (a loss)."

The past-participle adjectivesustained is attested by 1775 as "kept up or maintained uniformly," originally of music notes; the piano'ssustaining pedal is so called by 1889.