Definition of not know one's arse from one's elbowBritish, informal + impolite
- Don't take his word for it—he doesn't know his arse from his elbow.
: to perceive directly : have direct cognition of
: to have understanding of
: to recognize the nature of : discern
: the joint of the human arm
: a corresponding joint in the anterior limb of a lower vertebrate
: something (such as macaroni or an angular pipe fitting) resembling an elbow
: to push with the elbow : jostle
: to shove aside by pushing with or as if with the elbow
: to force (one's way) by pushing with or as if with the elbow
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