Thanks, Jackie. I had thought it was probably related to 'wall-eyed' but the source I tried, AHD online, didn't include the definition you cite as a possibility. Should have tried M-W. Wonder whether the same definition is in my 3D AHD.

Edit: Nope. My hardback AHD doesn't include the defintion. As many recalcitrant students as I've witnessed 'walling their eyes' or just 'walling' (vi.), I would think that the expression would have been picked up by AHD, too.