large hole; big opening
Literal physical hole or gap, often implying something is missing or damaged.
A large hole opened up in the road and it was closed to traffic.
big deficit; heavy losses
Financial or budgetary shortfall; often used in business or accounting contexts.
There is a huge deficit in this year's budget.
long-shot win; big winnings; a killing
Used in horse racing and other betting to describe an unexpected victory by an outsider, or the large payout from such a bet.
I hit a long-shot win at the horse races and got a lot of money.
本命 is the predicted top favorite, while 大穴 is a long-shot bet or outsider that would pay off big if it won.
Compound of 大 (big) and 穴 (hole). The financial and betting senses are metaphorical extensions of a large gap or opening.