thumb sucking
Refers to the habit of sucking one's thumb, typically by infants or young children.
指吸いは幼児によく見られる習慣です。
Thumb sucking is a common habit among toddlers.
Compound of 指 (yubi, 'finger') and 吸い (sui, 'sucking'), the nominalized form of 吸う (suu, 'to suck').