noun
A children's ritual where two people link little fingers while reciting a rhyme to seal a promise. The full rhyme is often 指切りげんまん、嘘ついたら針千本飲ます (Pinky swear, if you lie, you'll swallow a thousand needles).
See also: 指切りげんまん、嘘ついたら針千本飲ます
子供たちは指切りげんまんをして約束を交わした。
The children made a pinky swear to seal their promise.
「指切りげんまん、嘘ついたら針千本飲ます！」と二人は声を合わせた。
They chanted together, 'Pinky swear, if you lie, you'll swallow a thousand needles!'
指切り refers specifically to the act of linking little fingers to confirm a promise, while 指切りげんまん includes the accompanying rhyme and the full ritual.
The phrase is a set expression from children's folklore. 指切り (yubikiri) means 'finger-cutting' and refers to the gesture of linking little fingers. げんまん (genman) is said to be a corruption of 拳万 (kenman), meaning 'ten thousand fists', implying a severe punishment for breaking the promise. The exact origin is uncertain.