Middle English dereling, from Old English deorling, dyrling "one who is much beloved, a favorite," double diminutive of deor "dear" (see dear (adj.)). The vowel shift from -e- to -a- (16c.) is usual for -er- followed by a consonant (see marsh).
As an adjective "very dear, particularly beloved," from 1590s; in affected use, "sweetly charming" (1805). "It is better to be An olde mans derlyng, than a yong mans werlyng" (1562).