noun
palm of one's hand
Literal meaning: the inside surface of the hand. Often used in physical descriptions.
彼は手の内に小さな石を握っていた。
He held a small stone in the palm of his hand.
彼はなかなか手の内を見せない。
He doesn't easily reveal his true intentions.
試合では相手の手の内を読むことが大切だ。
In a match, it's important to read your opponent's strategy.
会社は彼の手の内にある。
The company is under his control.
noun
one's hand (in card games or mahjong)
Domain-specific use in mahjong and card games: the tiles or cards a player holds. Also used metaphorically in game contexts.
麻雀でいい手の内ができた。
I got a good hand in mahjong.
Compound of 手 (hand) + の (possessive particle) + 内 (inside). The literal meaning 'inside of the hand' extended to figurative senses of skill, control, and hidden intentions.