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to enter (house, room, etc.); to step in
彼は大胆に上がりこむ様子を見せた。
He boldly showed his manner of stepping in.
猫が突然家に上がりこんできた。
The cat suddenly stepped into the house.
雨の日には泥だらけで上がりこんだ。
On rainy days, I stepped in covered in mud.