Old English deorling "darling, favorite minion," double diminutive of deor "dear" (see dear (adj.)). The vowel shift from -e- to -a- (16c.) is usual for -er- followed by a consonant. "It is better to be An olde mans derlyng, than a yong mans werlyng" (1562).
Ps. 22:20; 35:17) means an "only one."