Why is the ninth month called September?
figurative sense is attested from 1967, from profit and loss accounting, where the final figure after both are calculated is the bottom line on the page. Also (especially as an adjective) bottomline.
The last line in an audit, which shows profit or loss.
Note: By extension, “bottom line” refers to the final, determining consideration in a decision.
Note: “Bottom line” also has a derogatory implication when it refers to those people whose attention to the bottom line prevents them from recognizing the value of anything else.