Picture a sheet of accounts. Line after line of money coming in and money going out, all the way down the page. Then one last line at the bottom with a single number on it: what is actually left. Everything above that line is detail and argument. That final line is the part you cannot talk your way around. Every use of bottom line borrows that feeling of one last line that settles the matter.
可以想成一張帳目表。上面一行一行寫著錢進來、錢出去，一路寫到頁面最下面。最後那一行只有一個數字：真正剩下的。那一行上面的全部都是細節，是可以吵的。最後那一行不能吵，它就是結果。bottom line 每一種用法，借的都是這個感覺：最下面那一行，講完就沒得談了。
From the word’s history
The source explains the phrase as a reference to the literal bottom line of an income statement or other accounting record.
來源說這個詞指的就是損益表或帳目紀錄上字面的「最後一行」。所以帳本的畫面不是我們自己想的，是來源直接寫的。
Wiktionary, "bottom line"
登入就可以看每一支怎麼連回核心畫面。