指切りげんまん、嘘ついたら針千本飲ます

ゆびきりげんまん、うそついたらはりせんぼんのますyubikirigenman、usotsuitaraharisenbonnomasu

expression

1. cross my heart and hope to die, stick a needle in my eye
2. pinky promise pledge: if you're lying, I'll make you drink a thousand needles

Study it in Yanamino