1.
snugly warm, cozily
Comfortably warm, often in a sheltered environment.
2.
living in comfort without hardship
Living an easy life without experiencing difficulties (often with slight negative nuance).
ぬくぬく is an onomatopoeia describing cozy warmth or comfortable living.
Sense 1 - physical warmth
SENSE 2 - SHELTERED LIFE (often negative):
Verbal use
ぬくぬくする: to be cozy, to snuggle up
Similar words
Note: Sense 2 can carry a critical nuance, implying someone is too comfortable and hasn't experienced real difficulties.
Related Words
Related:
ぽかぽか (warm (from the sun))
Related:
ほかほか (steaming hot (food))
Compare:
あったかい (warm)
Compare:
暖かい (warm)