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.

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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), thunder (n.), pound (v.), 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. Also obsolete round (n.) "whispering," variant of roun "secret, mystery, divine mystery" (from Old English run, source of rune), with unetymological -d attested by 15c. in the verb. 

Among the words from French are powder (n.), meddle, tender (adj.), remainder, gender (n.), 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.

c. 1300, astonien, "to stun, strike senseless," from Old French estoner "to stun, daze, deafen, astound," from Vulgar Latin *extonare, from Latin ex "out" (see ex-) + tonare "to thunder" (see thunder (n.)); so, literally "to leave someone thunderstruck." The modern form (influenced by English verbs in -ish, such as distinguish, diminish) is attested from 1520s. The meaning "amaze, shock with wonder" is from 1610s.

No wonder is thogh that she were astoned [Chaucer, "Clerk's Tale"]

In Tyndale the voice of God left Saul astonyed. Related: Astonished; astonishing.

"short, large-bore gun or firearm with a funnel-shaped muzzle," 1650s, from Dutch donderbus, from donder "thunder" (Middle Dutch doner, donder, from Proto-Germanic *thunaraz; see thunder (n.)) + bus "gun" (originally "box, tube"); altered by resemblance to blunder. Related: Blunderbussier.

1670s, "explosion accompanied by loud sound," from French détonation, from Medieval Latin detonationem (nominative detonatio), from Latin detonare "to thunder down, to release one's thunder, roar out," from de "down" (see de-) + tonare "to thunder" (see thunder (n.)). Sense of "act of causing to explode" (mid-18c.) developed in French.

"to thunder, rumble," 1620s, from past participle stem of Latin intonare "to thunder, thunder forth," from in- (from PIE root *en "in") + tonare "to thunder" (see thunder (n.)). Related: Intonated; intonating.

obsolete 17c.-18c. verb, from French entoner "thunder, roar, resound, reverberate," from Latin intonare "to thunder, resound," figuratively "to cry out vehemently," from tonare "to thunder" (see thunder (n.)). Related: Intoned; intoning.

thunder-god of Norse mythology, Odin's eldest son, strongest of the gods and not the wisest, c. 1020, from Old Norse Þorr, literally "thunder," from *þunroz, related to Old English þunor (see thunder (n.)). His weapon was the hammer Mjölnir (said to be literally "crusher" but there are other theories).

1833, U.S. colloquial exclamation, by thunderation! From 18c.-19c. use of thunder (n.) in oaths, in which it was often paired with damnation. Also compare tarnation.

legendary cause of thunder (by the flapping of its wings) in some Native American cultures, 1848, a translation of native words, such as Ojibwa (Algonquian) aninikii, Lakotah (Siouan) wakiya, Klamath /lmelmnis/. See thunder (n.) + bird (n.1). In Lakhota, "the thunderbirds call" is "the usual expression for thunder" [Bright].

"flash of lightning with the accompanying crash of thunder," mid-15c., from thunder (n.) + bolt (n.) "arrow, projectile." Originally especially in reference to the lightning, as an imaginary bolt or shaft hurled by Zeus or Jupiter. Figuratively, "one who is bold and irresistible," from 1590s.