leer 
(lîr)
(lîr)intr.v. leered, leer·ing, leers
To look with a sidelong glance, indicative especially of sexual desire or sly and malicious intent.
n.
A desirous, sly, or knowing look.
[Probably from obsolete leer, cheek, from Middle English ler, from Old English hlēor; see kleu- in the Appendix of Indo-European roots.]
leering·ly adv.