The Collaborative International Dictionary
Smothered mate \Smoth"ered mate\ (Chess) Checkmate given when movement of the king is completely obstructed by his own men. This can only occur when an opponent moves a knight into a position to give a check to the king.
Wiktionary
n. (context chess English) A checkmate delivered by a knight in which the mated king is unable to move because he is surrounded (or ''smothered'') by his own pieces.
Wikipedia
In chess, a smothered mate is a checkmate delivered by a knight in which the mated king is unable to move because he is surrounded (or smothered) by his own pieces.
The mate is usually seen in a corner of the board, since fewer pieces are needed to surround the king there. The most common form of smothered mate is seen in the diagram to the right. The knight on f7 delivers mate to the king on h8 which is prevented from escaping the check by the rook on g8 and the pawns on g7 and h7. Similarly, White can be mated with the white king on h1 and the knight on f2. Analogous mates on a1 and a8 are rarer, because kingside castling is the more common as it safely places the king closer to the corner than it would have the castling occurred on the queenside.