noun
A guess or hunch, often based on intuition or experience rather than solid evidence. Frequently used in phrases like 山勘が当たる (the guess is right) or 山勘で言う (say based on a hunch).
彼の山勘が当たって、宝くじに当選した。
His hunch was right, and he won the lottery.
山勘で答えたら、たまたま正解だった。
I answered based on a guess, and it happened to be correct.
Compound of 山 (yama, 'mountain') and 勘 (kan, 'intuition'). The exact origin is uncertain, but it may derive from the idea of a prospector's guess about where to find ore in a mountain, or from gambling slang.