"fly up suddenly," c.1300, perhaps imitative of the sound of beating wings, or related to flash via its variant flushe. Probably not connected to Old French flux, source of flush (n.).
Transitive meaning "to cause to fly, start" is first attested mid-15c. The sense of "spurt, rush out suddenly, flow with force" (1540s) is probably the same word, with the connecting notion being "sudden movement," but its senses seem more to fit the older ones of flash (now all transferred to this word except in flash flood). Meaning "cleanse a drain, etc., with a rush of water" is from 1789. The noun sense of "sudden redness in the face" (1620s) probably belongs here, too. The verb in this sense is from 1660s. "A very puzzling word" [Weekley]. Related: Flushed; flushing.
1550s, "perfect, faultless;" c.1600, "abundant; plentifully supplied (with money, etc.)," perhaps from flush (v.) through the notion of a river running full, hence level with its banks. Meaning "even, level" is from 1620s.
"hand of cards all of one suit," 1520s, perhaps from Middle French flus (15c.), from Old French flux "a flowing," with the sense of "a run" (of cards), from Latin fluxus "flux," from fluere "to flow" (see fluent). The form in English probably was influenced by flush (v.).
flush 1 (flŭsh)
v. flushed, flush·ing, flush·es
To turn red, as from fever, heat, or strong emotion; blush.
To clean, rinse, or empty with a rapid flow of a liquid, especially water.
An act of cleansing or rinsing with a flow of water.
A reddening of the skin, as with fever, emotion, or exertion.
A brief sensation of heat over all or part of the body.
Having plenty of money; affluent, esp temporarily; rich: It took money, and the jazzman wasn't ever too flush (1603+)
four-flush, in a flush