c. 1300, "delightful to the senses, pleasing in the highest degree" (implied in deliciously), from Old French delicios (Modern French délicieux), from Late Latin deliciosus "delicious, delicate," from Latin delicia (plural deliciae) "a delight, allurement, charm," from delicere "to allure, entice," from de- "away" (see de-) + lacere "to lure, entice," which is of uncertain origin.
Especially, but not exclusively, of taste. Related: Deliciously. As a name of a type of apple, attested from 1903, first grown by Jesse Hiatt of Iowa, U.S.A. Colloquial shortening delish is attested from 1920.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).