Source : Webster's Revised Unabridged Dictionary (1913)

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.

         I will leerupon him as a' comes by.      --Shak.

         The priest, above his book, Leering at his neighbor's
         wife.                                    --Tennyson.

Source : WordNet®

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"