[請益] from their elbow

[hubertt (彼岸花)]

請問from their elbow是什麼意思呢？
我查了urban和reference dictionary都查不到，

網站的標題就是：
They don't know their anus from their elbow - literally

內文還有：
I have long believed that most of the academics writing about German Jews don't
know their behinds from their elbows

附上網站網址
http://0rz.tw/DFsjB

謝謝~

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※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 218.171.57.25
→ dunchee:http://www.ldoceonline.com/dictionary/know_1
→ dunchee:-> 14 ...
 請問固定跟elbow搭配，因為分辨不出xxxx和手肘有什麼不同，
 是暗指「很愚蠢」的意思嗎？
 我還有在別的文章看到：
 most British people truly do not know their apostrophe from their elbow..

※ 編輯: hubertt         來自: 218.171.57.25        (11/05 12:18)
推 pttlad:know=tell, means stupid
→ hubertt:感謝D大和P大，我大概理解了^^~