The Collaborative International Dictionary of English v.0.48:

Leer \Leer\, v. i. [imp. & p. p. Leered (l[=e]rd); p. pr. &
   vb. n. Leering.]
   To look with a leer; to look askance with a suggestive
   expression, as of hatred, contempt, lust, etc.; to cast a
   sidelong lustful or malign look.
   [1913 Webster]

         I will leerupon him as a' comes by.      --Shak.
   [1913 Webster]

         The priest, above his book,
         Leering at his neighbor's wife.          --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

leering
    adj 1: showing sly or knowing malice in a glance; "she had run
           in fear of...his evil leering eye"- Amy Lowell
    2: (of a glance) sidelong and slyly lascivious; "leering
       drugstore cowboys"