"supporting oneself or itself without extraneous help," 1650s, from self- + present participle of sustain (v.). Related: Self-sustained (1742).
Entries linking to self-sustaining
self-
word forming element indicating "oneself," also "automatic," from Old English use of self (pron.) in compounds, such as selfbana "suicide," selflice "self-love, pride, vanity, egotism," selfwill "free will." Middle English had self-witte "one's own knowledge and intelligence" (early 15c.).
OED counts 13 such compounds in Old English. Middle English Compendium lists four, counting the self-will group as a whole. It re-emerges as a living word-forming element mid-16c., "probably to a great extent by imitation or reminiscence of Greek compounds in (auto-)," and formed a great many words in the pamphlet disputes of the 17c.
sustain v.
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.