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.
c. 1300, "give support to," from stem of Old French sostenir "hold up, bear; suffer, endure" (13c.), from Latin sustinere "hold up, hold upright; furnish with means of support; bear, undergo, endure," from assimilated form of sub "up from below" (see sub-) + tenere "to hold," from PIE root *ten- "to stretch." Meaning "continue, keep up" (an action, etc.) is from early 14c. Sense of "endure without failing or yielding" is from c. 1400. Related: Sustained; sustaining.
<a href="https://www.etymonline.com/word/self-sustaining">Etymology of self-sustaining by etymonline</a>
Harper, D. (n.d.). Etymology of self-sustaining. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/self-sustaining