also sun-down, 1610s, from sun (n.) + down (adv.). OED suggests perhaps a shortening of sun-go-down (1590s). Compare sunset.

"a stepping down, descent" (obsolete), late 15c., from Latin degressionem (nominative degressio) "a going down," noun of action from past-participle stem of degredi "to go down, march down, descend," from de- "down" (see de-) + gradi "to walk, go, step" (from PIE root *ghredh- "to walk, go").

also wash-down, 1949, from verbal phrase, from wash (v.) + down (adv.).

early 15c., perfusen, "to wash away;" 1520s, "to sprinkle, pour or spread over or through," from Latin perfusus, past participle of perfundere "to pour over, besprinkle," from per (see per) + fundere "to pour, melt" (from nasalized form of PIE root *gheu- "to pour").

1680s, "to melt, make liquid by heat" (transitive), back-formation from fusion. Intransitive sense, "to become liquid," attested from 1800. Figurative sense of "blend different things, blend or unite as if by melting together" is recorded by 1817. Intransitive figurative sense "become intermingled or blended" is by 1873. Related: Fused; fusing.

also slow-down, 1892, "act of going more slowly," from the verbal phrase; see slow (v.) + down (adv.). To slow up "slow down speed" is attested by 1881.

early 15c., demitten, "to run or flow down," also figurative, "to humble oneself," from Old French demetre "to send, put, or let down," and directly from Latin demittere "to send down," from de "down" (see de-) + mittere "to release, let go; send, throw" (see mission).

of winds, "blowing down a slope," 1904, from Greek katabatos "descending," from katabainein "to go down," a compound of kata "down" (see cata-) + bainein "to go, walk, step," from PIE root *gwa- "to go, come."

"leaning, reclining," 1705, from Latin recumbentem (nominative recumbens), present participle of recumbere "recline, lie down, lie down again;" of things, "to fall, sink down, settle down," from re- "back" (see re-) + -cumbere "to lie down" (related to cubare "lie down;" see cubicle). Related: Recumbency (1640s); recumbence (1670s). A verb, recumb, has been attempted in English occasionally since 1670s.

"barricade defense made of felled trees with the branches angled outward," 1766, from French abatis, literally "things thrown down," from Old French abateiz "a casting down; slaughter, carnage" (12c.), from abatre "to beat down, throw down" (see abate).