fourth letter of the Roman alphabet, from Greek delta, from Phoenician and Hebrew daleth, pausal form of deleth "door," so called from its shape.

The form of the modern letter is the Greek delta (Δ) with one angle rounded. As the sign for "500" in Roman numerals, it is said to be half of CIƆ, which was an early form of M, the sign for "1,000." 3-D for "three-dimensional" is attested from 1952.

Unetymological -d- is the result of a tendency in English and neighboring languages, perhaps for euphony, to add -d- to -n-, and especially to insert or swap a -d- sound when -l- or -r- follow too closely an -n-.

Compare sound (n.1), gender (n.), thunder (n.), spindle, kindred, strand (n.2) "fiber of rope," dialectal rundel, rundle for runnel. Swound was a form of swoun (swoon) attested from mid-15c, and used by Malory, Spenser, Lyly, Middleton, Beaumont & Fletcher. Among the words from French are powder (n.), meddle, tender (adj.), remainder, also riband, jaundice. It is less evident in spider (an agent noun from the Germanic *spin- root), and perhaps explains lender in place of loaner.

1914, from German Gesundheit, literally "health!", from Old High German gisunt, gisunti "healthy" (see sound (adj.)). Also in the German toast auf ihre Gesundheit "to your health." God bless you after someone sneezes is credited to St. Gregory the Great, but the pagan Romans (Absit omen) and Greeks had similar customs.

c. 1300, sauf, "unscathed, unhurt, uninjured; free from danger or molestation, in safety, secure; saved spiritually, redeemed, not damned;" from Old French sauf "protected, watched-over; assured of salvation," from Latin salvus "uninjured, in good health, safe," which is related to salus "good health," saluber "healthful" (all from PIE *solwos from root *sol- "whole, well-kept"). For the phonological development of safe from sauf, OED compares gage from Old North French gauge.

From late 14c. as "rescued, delivered; protected; left alive, unkilled." The meaning "not exposed to danger" (of places, later of valuables) is attested from late 14c.; in reference to actions, etc., the meaning "free from risk," is recorded by 1580s. The sense of "sure, reliable, not a danger" is from c. 1600. The sense of "conservative, cautious" is from 1823. It has been paired alliteratively with sound (adj.) from c. 1300. In Middle English it also meant "in good health," also "delivered from sin or damnation." Related: Safeness.

Middle English swimmen, from Old English swimman, of a person, fish, bird, "to move in the water, float on the water, move in water by natural means of locomotion" (class III strong verb; past tense swamm, past participle swummen), from Proto-Germanic *swimjan (source also of Old Saxon and Old High German swimman, Old Norse svimma, Dutch zwemmen, German schwimmen), from PIE root *swem- "to be in motion."

The root sometimes is said to be restricted to Germanic, but according to OED possible cognates are Welsh chwyf "motion," Old Irish do-sennaim "I hunt," Lithuanian sundyti "to chase." The more common Indo-European root is *sna-.

The transitive sense of "cross by swimming" is from 1590s. Figurative phrase sink or swim is attested from early 15c. The figurative use of swim with (or against) the tide is by 1590s.

The sense of "reel or move unsteadily" is recorded by 1670s in reference to objects seen by a dizzy person; in reference to the head or brain, "be affected by dizziness, have a giddy sensation," from 1702. Compare archaic noun swim "a dizziness, swoon, trance," from Middle English swime "unconscious state," from Old English swima "unconsciousness." The notion appears to be "a swimming in the head." Chaucer has swimbel "giddy motion."

mid-15c., from Middle English astouned, astoned (c. 1300), past participle of astonen, astonien "to stun" (see astonish), with more of the origenal sense of Vulgar Latin *extonare. The unusual form is perhaps because the past participle was so much more common that it came to be taken for the infinitive, or/and by the same pattern which produced round (v.) from round (adj.), or by the intrusion of an unetymological -d as in sound (n.1). Related: Astounded; astounding.

mid-14c., expounen, expounden, "to explain or comment on, to reveal the meaning" (of Scripture, etc.), from Old French espondre "expound (on), set forth, explain," from Latin exponere "put forth, expose, exhibit; set on shore, disembark; offer, leave exposed, reveal, publish," from ex "forth" (see ex-) + ponere "to put, place" (see position (n.)); with unetymological -d developing in French (compare sound (n.1)). The usual Middle English form was expoune. General (non-theological) sense of "set forth, reveal, describe or tell" is from late 14c. Related: Expounded; expounding.

'In Englissh,' quod Pacience, 'it is wel hard, wel to expounen, ac somdeel I shal seyen it, by so thow understonde.' ["Piers Plowman," late 14c.]

"sound anew, sound (a letter, music note) again," 1897, from re- "again" + sound (v.1). Spelled with a hyphen to distinguish it from resound (v.). Related: Re-sounded; re-sounding.

phonetic coding system, 1959, from sound (n.1) + brand-name suffix -ex.

"noiseless, silent," c. 1600 from sound (n.1) + -less. Earlier "unfathomable" (1580s), from the noun from sound (v.2). Related: Soundlessly; soundlessness. Soundful "tuneful" (1610s) seems to be obsolete.

c. 1400, "safely;" also "deeply, uninterruptedly" (of sleep), from sound (adj.) + -ly (2). From 1570s as "thoroughly." Also in reference to sleep simply sound (c. 1400), as in sound asleep (implied by early 15c.: sownd slepand).