to look with a sideways or oblique glance, esp. 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; perh. v. use of obs. leer cheek (ME leor, OE hlēor; c. ON hlȳr (pl.))
leer (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 Indo-European roots.] leer'ing·ly adv.
"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.