To lose everything; to suffer a total loss so that both the principal and any gains are gone; to end up with nothing.
借金をしてギャンブルに使えば元も子もない。
If you borrow money and spend it on gambling, you'll end up losing everything.
事業を拡大しすぎて失敗すると、元も子もない結果になることがある。
If you expand a business too much and it fails, you can end up with nothing at all.
賭けに出て成功すればいいが、失敗したら元も子もない。
It's fine to take a gamble if you succeed, but if you fail you'll lose everything.