From Hard Drop - Tetris Wiki

In multiplayer games, to show bad manners or BM is to disrespect the opponent.

Common BM methods include:

• Spamming the hard drop key;
• Soft dropping every piece;
• Using impractical openers such as the Sus Opener;
• Taunting the opponent in chat.

As the term implies, it is generally considered rude to BM, especially in ranked or tournament matches where serious play is expected.