not know one's arse from one's elbow

: to perceive directly : have direct cognition of

: to have understanding of

: to recognize the nature of : discern

: knowledge

: 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