substantiate v.

c. 1300, "essential nature, real or essential part," from Old French sustance, substance "goods, possessions; nature, composition" (12c.), from Latin substantia "being, essence, material," from substans, present participle of substare "stand firm, stand or be under, be present," from sub "up to, under" (see sub-) + stare "to stand," from PIE root *sta- "to stand, make or be firm."

Latin substantia translates Greek ousia "that which is one's own, one's substance or property; the being, essence, or nature of anything." Meaning "any kind of corporeal matter" is first attested mid-14c. Sense of "the matter of a study, discourse, etc." first recorded late 14c.

1760, "embodiment;" 1832, "the making good of a statement, the act of proving," noun of action from substantiate.

1775, from un- (1) "not" + past participle of substantiate (v.).