to look with a sideways or oblique glance, especially suggestive of lascivious interest or sly and malicious intention: I can't concentrate with you leering at me.
noun
2.
a lascivious or sly look.
Origin: 1520–30; perhaps v. use of obsolete leer cheek (Middle Englishleor,Old Englishhlēor; cognate with Old Norsehlȳr (plural))
"to look obliquely" (now usually implying "with a lustful or malicious intent"), 1530, from M.E. noun ler "cheek," from O.E. hleor "the cheek, the face," from P.Gmc. *khleuzas "near the ear," from *kleuso- "ear," from PIE root *kleu- "to hear" (see listen). The notion is
probably of "looking askance" (cf. figurative development of cheek). The noun is first attested 1598.