sustenance n.

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.