To slip or sneak into a place stealthily; to creep in unnoticed.
泥棒が夜中に家に忍び込んだ。
A thief sneaked into the house in the middle of the night.
彼は試験会場に忍び込もうとして見つかった。
He was caught trying to sneak into the exam hall.
子どもたちは大人に見つからないように図書館に忍び込んで本を読んだ。
The children snuck into the library so they wouldn't be seen by adults and read books.