c. 1400, "faint with heat," frequentative of swelten "be faint (especially with heat)," late 14c., from Old English sweltan "to die, perish," from Proto-Germanic *swiltan- (source also of Old Saxon sweltan "to die," Old Norse svelta "to put to death, starve," Gothic sviltan "to die"), perhaps originally "to burn slowly," hence "to be overcome with heat or fever," from PIE root *swel- (2) "to shine, beam" (see Selene). From the same ancient root comes Old English swelan "to burn." For specialization of words meaning "to die," compare starve. Related: Sweltered; sweltering.

adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy). Adjectives such as hugy, vasty are artificial words that exist for the sake of poetical metrics.

1570s, for *sweltery, from swelter (v.) + -y (2).

a name of the moon goddess, equivalent to Latin Luna, from Greek selēnē "the moon; name of the moon goddess," related to selas "light, brightness, bright flame, flash of an eye." This is reconstructed to be from PIE root *swel- (2) "to shine, beam" (source also of Sanskrit svargah "heaven," Lithuanian svilti "to singe," Old English swelan "to be burnt up," Middle Low German swelan "to smolder") and to be related to swelter and sultry.

Daughter of Hyperion and Theia, sister of Helios. Related: Selenian "of or pertaining to the moon as a world and its supposed inhabitants," 1660s. Another early word for "moon-man, supposed inhabitant of the moon" is selenite (1640s); Greek had selēnitai "moon-dwellers, the men in the moon" (Lucian).