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"to look obliquely" (now usually implying "with a lustful or malicious intent"), 1520s, probably from Middle English noun ler "cheek," from Old English hleor "the cheek, the face," from Proto-Germanic *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). Related: Leered; leering.
1590s, from leer (v).