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

: variant ofass

:  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