該語的字面意思是“大拇指的規則”，實際意義為“（根據實際經驗和實踐的）對事物粗略但實用的估計方法”。其出典有兩種說法：一說源自舊時用拇指大略計算長度的做法；另一種說法源自昔日釀酒工人將拇指浸在所釀酒中估計溫度的做法。

A: Do you think it’s a good idea to buy some traveler’s checks? 你認為買旅行支票是個好辦法嗎？
B: Definitely! Don’t carry large amounts of cash with you when you’re traveling. We have a rule of thumb: never carry around any more cash than you can afford to have stolen. To be on the safe side, you’d better ask for smaller denominations: $20 or $50. 沒錯。旅行時不要隨身帶大額現金。我們有個經驗之法：隨身攜帶的錢要適量，丟了不要緊。為了穩妥起見，你最好要小面額的：20或者元。
A: I was told that if you use traveler’s checks, you don’t countersign the check until the clerk in the store is watching you. 我聽說在使用旅行支票時，要當著商店售貨員的面簽字。
B: Yes, you’ve got it. 你說得對.

來源:可可英語 http://www.ccdyzl.cn/kouyu/201003/99319.shtml