Other forms: leering, leers
To leer is to look at somebody in an unpleasant knowing way. A leering grin, leered at her, an unpleasant leer.
It is nearly always sexual and always unwelcome, which is what separates it from staring — a leer carries an implication. The sideways quality is in the word's history: it appears to come from an old noun meaning cheek or face, so leering was turning the face aside to look. Note leery, which is a different word meaning wary.
1verbleerTo look at somebody in an unpleasant knowing way.
“The drunk man leered at the women passing his table.”
leer is a way of looking.
Apparently from an old verb meaning to look sideways, from a noun leer meaning cheek or face. Leery, meaning wary, is a separate word.
Built from
Exact development uncertain, but apparently from a verb leer (“to make a face, look sideways”), from leer#Etymology_2 (“cheek, face, profile”).
Etymology from Wiktionary, CC BY-SA
Practised in Ways of Looking