flush (v.1)

mid-13c., flusshen "move rapidly or violently; rush, dart, spring" (intransitive); late 15c., flush up, transitive, "cause to fly; start or flush (birds)," perhaps imitative of the sound of beating wings.

The sense of "spurt, rush out suddenly, flow with force" (1540s, usually of water) probably is the same word, with the connecting notion being "sudden movement," but its senses seem more to fit the older ones of flash (v.), now all transferred to this word except in flash flood, via its variant flushe. OED considers this probably not connected to Old French flux. Transitive sense "cause to flow" is from 1590s.

Meaning "cleanse (a drain, etc.) with a rush of water" is from 1789. Of the face, "become suffused with warm color," from 1680s (flushed). Sense of "inflame with pride or passion" as a result of success, victory, etc., is from 1630s; perhaps influenced in sense by flesh (v.). Related: Flushed; flushing.

flush (adj.)

1550s, "perfect, faultless;" c. 1600, "abundantly full," also "full of life or spirit," also "plentifully supplied" (with money, etc.), perhaps from flush (v.1) through the notion of a river running full, hence level with its banks. Meaning "even, level" is from 1620s, originally of ship's decks. In general use by 1791; in typography, 1900; in pugilism, 1812.

flush (n.)

The section of entries for the various flushes in Century Dictionary opens with a caveat:

The several words spelled flush, being mostly dialectal, colloquial, or technical, and scantily recorded in early literature, have become partly confused with one another, and cannot now be entirely disentangled. Words originally different have acquired some meanings very nearly identical, while on the other hand there are some meanings not obviously related which are, nevertheless, to be referred to one original.

Weekley calls it "A very puzzling word." Sense of "a rush of water" in a stream (1520s), is probably from flush (v.1). From this likely come the extended senses "rush of emotion or passion" (1610s); "a sudden shooting up" (1773); "act of cleansing (a drain) by flushing" (1883); "glow of light or color" (especially sudden redness in the face), 1620s. Independently from the verb, probably, is the noun sense of "a flight of birds suddenly started up" (1590s).

The meaning "hand of cards all of one suit" (1520s) is of uncertain origin, perhaps formed on the model of French flus (15c.), from Old French flux, flus "a flowing, rolling" (see flux), which, in common with its Italian cognate flusso, is said to have once had a sense of "a run" of cards. The form in English probably was influenced by flush (v.1).

flush (adv.)

"directly, straight," 1700, from flush (adj.).

flush (v.2)

"make even or level," 1842, from flush (adj.).

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Definitions of flush from WordNet
flush (v.)
turn red, as if in embarrassment or shame;
Synonyms: blush / crimson / redden
flush (v.)
flow freely;
The garbage flushed down the river
flush (v.)
glow or cause to glow with warm color or light;
the sky flushed with rosy splendor
flush (v.)
make level or straight;
Synonyms: level / even out / even
flush (v.)
rinse, clean, or empty with a liquid;
flush the wound with antibiotics
Synonyms: scour / purge
flush (v.)
irrigate with water from a sluice;
Synonyms: sluice
flush (v.)
cause to flow or flood with or as if with water;
flush the meadows
flush (n.)
the period of greatest prosperity or productivity;
flush (n.)
a rosy color (especially in the cheeks) taken as a sign of good health;
Synonyms: bloom / blush / rosiness
flush (n.)
sudden brief sensation of heat (associated with menopause and some mental disorders);
Synonyms: hot flash
flush (n.)
a poker hand with all 5 cards in the same suit;
flush (n.)
the swift release of a store of affective force;
Synonyms: bang / boot / charge / rush / thrill / kick
flush (n.)
a sudden rapid flow (as of water);
he heard the flush of a toilet
Synonyms: gush / outpouring
flush (n.)
sudden reddening of the face (as from embarrassment or guilt or shame or modesty);
Synonyms: blush
flush (adv.)
squarely or solidly;
hit him flush in the face
flush (adv.)
in the same plane;
set it flush with the top of the table
flush (adj.)
of a surface exactly even with an adjoining one, forming the same plane;
the bottom of the window is flush with the floor
a door flush with the wall
flush (adj.)
having an abundant supply of money or possessions of value;
a speculator flush with cash