also: みせパイ
noun
exposed tile; tile shown by mistake
In mahjong, a tile that a player accidentally reveals to others, often resulting in a penalty or disadvantage.
見せ牌をしてしまい、相手に手の内がばれた。
I accidentally showed a tile, and my opponents figured out my hand.
ルールによっては、見せ牌は罰則の対象になる。
Depending on the rules, an exposed tile may be subject to a penalty.
Compound of 見せ (mise, 'showing') + 牌 (hai, 'tile'), literally 'show tile'.