One way to picture it: you have poured what you need into the container, and there is still some left over the top of it — the part beyond the line of 'enough'.
From the word’s history
From Middle English surplus, from Middle French surplus: sur- "over, above" (Latin super) + plus "more" — literally what is over and above.
經由中古英語 surplus 借自中古法語 surplus：sur-（在上面、超過，來自拉丁 super）+ plus（更多），字面就是「超出去的那些」。這解釋了為什麼 surplus 一定要有一個對照的基準——沒有「需要多少」，就談不上「多出多少」。
Middle English surplus < Middle French surplus, as given in the entry's attested origin note
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原始詞典資料6 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
surplusnoun/ˈsɝˌplʌs/
- 1
That which remains when use or need is satisfied, or when a limit is reached; excess; overplus; overage.
- 2
Specifically, an amount in the public treasury at any time greater than is required for the ordinary purposes of the government.
- 3
lawThe remainder of a fund appropriated for a particular purpose.
- 4
lawassets left after liabilities and debts, including capital stock have been deducted.
surplusadj/ˈsɝˌplʌs/
- 1
Being or constituting a surplus; more than sufficient.
- surplus population
- surplus words
surplusverb/ˈsɝˌplʌs/
- 1
To treat as surplus to requirements; to sell off or dismiss from employment, etc.
- This employee was engaged to direct asphalt plants and inasmuch as the work for which he had been employed was completed, he was surplused and his return travel was approved […]
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。