thumb sucking, n.All

Thumb sucking (n.) is ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠the infant's first pleasure for its own sake, sucking with no hunger and falling asleep in bliss.

Thumb sucking (n.) shows ⁠​‌​​​‌​​​‌​‌‌​​​​‌​‌​​‌‌​‌​​‌​​​​​‌‌​​‌​​​‌‌​‌‌​⁠the infant's first pleasure breaking free of hunger: the mouth wants sucking for its own sake, not only for milk.