Definition:
: to cast a sidelong glance : give a lascivious, knowing, or malicious look
About the Word:
All dictionaries will occasionally hedge their bets when giving the history of a word; the science of etymology is imprecise enough that if a dictionary's compilers had to leave out uncertain origins the books they create would be considerably smaller.
So the roots of leer are referred to with some degree of circumspection; it probably comes from the noun form of leer, which is an obsolete word for the cheek, since the earliest uses of leer as a verb seem to indicate the action of a sidelong glance, or looking over one's cheek. If you have a better idea of where it comes from, feel free to write to your local dictionary to tell them of it. Just kidding. Don't do that.
Example:
"And she leered at me with long, odious eyes — glances freighted with impurity." — S.C. Hall, in Tales of the Day, Volume 1, 1861
