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.

c. 1200, from Old English cynn "family; race; kind, sort, rank; nature" (also "gender, sex," a sense obsolete since Middle English), from Proto-Germanic *kunja- "family" (source also of Old Frisian kenn, Old Saxon kunni "kin, kind, race, tribe," Old Norse kyn, Old High German chunni "kin, race;" Danish kjön, Swedish kön, Middle Dutch, Dutch kunne "sex, gender;" Gothic kuni "family, race," Old Norse kundr "son," German Kind "child"), from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups.

In the Teutonic word, as in Latin genus and Greek [ genos], three main senses appear, (1) race or stock, (2) class or kind, (3) gender or sex .... [OED]

Related to both words kind and to child. From 1590s as an adjective, from the noun and as a shortening of akin. Legal next of kin (1540s) does not include the widow, "she being specifically provided for by the law as widow" [Century Dictionary], and must be a blood relation of the deceased.