in early use also engredient, early 15c., "something forming part of a mixture," from Latin ingredientem (nominative ingrediens) "that which enters into" (a compound, recipe, etc.), present participle of ingredi "go in, enter," from in- "in" (from PIE root *en "in") + gradi "to step, go" (from PIE root *ghredh- "to walk, go"). Also from early 15c. as an adjective, "forming part of a mixture."

Proto-Indo-European root meaning "to walk, go." 

It forms all or part of: aggress; aggression; aggressive; centigrade; congress; degrade; degree; degression; digress; digression; egress; gradation; grade; gradual; graduate; grallatorial; gravigrade; ingredient; ingress; plantigrade; progress; progression; regress; regression; retrograde; retrogress; tardigrade; transgress; transgression.

It is the hypothetical source of/evidence for its existence is provided by: Latin gradus "a step, a pace, gait," figuratively "a step toward something, a degree of something rising by stages;" gradi "to walk, step, go;" Lithuanian gridiju, gridyti "to go, wander;" Old Church Slavonic gredo "to come;" Old Irish in-greinn "he pursues."  

toffee-like confection, 1802, from butter (n.), which is a main ingredient; the second element uncertain; perhaps from its having been made in Scotland.

"thick, whitish fluid containing spermatozoa as its essential ingredient," late 14c., from Latin semen "seed of plants, animals, or men; race, inborn characteristic; posterity, progeny, offspring," figuratively "origin, essence, principle, cause" (from PIE *semen- "seed," suffixed form of root *sē- "to sow").

also corn-flakes, cornflakes, type of breakfast cereal, 1902, from corn (n.1) + flake (n.); the process of making them was discovered by American food manufacturer W.K. Kellogg in 1894. Earlier corn flakes was an ingredient in brewing (1890s).

"fried spring roll," a Chinese-American food, by 1917, though the ingredients have changed, from egg (n.) + roll (n.). The reason for egg is unclear now, as they often contain no egg and cabbage is the primary ingredient, but in the old recipe the shell they were rolled in was made of fried eggs.

c. 1600, "to fill with an ingredient, spirit, etc.;" 1640s as "make (a female) pregnant," from Late Latin impraegnatus "pregnant," past participle of impraegnare "to render pregnant," from assimilated form of in- "into, in" (from PIE root *en "in") + praegnare "make pregnant" (see pregnant). Earlier in same sense was impregn (1530s), which OED marks as "now only in poetic use."