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to creep in; to steal in; to sneak in
泥棒どろぼうが夜中よなかに忍び込んしのびこんだ。
A thief snuck in during the night.
猫ねこが部屋へやに忍び込んしのびこんでいた。
The cat had snuck into the room.
子供こどもの頃ころに友達ともだちの家いえに忍び込むしのびこむ遊びあそびをした。
As a child, I played at sneaking into friends' houses.