betting on an outsider; long shot betting; dark horse betting
Used primarily in horse racing and similar gambling contexts to describe betting on a competitor with low expected odds. Can also be used figuratively for taking a chance on something unlikely.
He always bets on outsiders in horse racing.
Long shot investments carry high risk.
Compound of 穴 (あな, 'hole', here metaphorically 'outsider' or 'dark horse') and 狙い (ねらい, 'aiming'). The 穴 metaphor comes from the idea of an unexpected gap or hidden opportunity.