godan verb (-ru), intransitive verb
jump in; dive in; rush in
Rare intransitive verb meaning to enter suddenly or forcefully, often with a sense of leaping or plunging into a place or situation. More common synonyms like 飛び込む are usually preferred.
彼はためらわずに川へ飛び入った。
He jumped into the river without hesitation.
突然
Suddenly, he rushed into the room.