two pawns on the same file; illegal double pawn drop
Shogi term for the illegal move of dropping a second unpromoted pawn onto a file that already contains one of the player's unpromoted pawns.
Making a double pawn drop results in a loss by foul.
Beginners sometimes accidentally make an illegal double pawn drop.
Another illegal shogi move: dropping a pawn to give checkmate. 二歩 is specifically about having two unpromoted pawns on the same file.
Compound of 二 (two) and 歩 (pawn, foot soldier). The reading にふ follows the on'yomi of both characters.