Leer
/lɪr/
Definition 1 of 2
Pro Tip 1/3Negative Connotation
Using 'leer' implies disapproval and suggests the look is inappropriate. 
Her boss's constant leering made her uncomfortable.
Pro Tip 2/3Visual Imagery
'Leer' often conveys a picture of someone with a suggestive or malicious grin. 
As the villain leered at the hero, his malicious intent was obvious.
Pro Tip 3/3Unwelcome Attention
Leering is unwelcome and can make the recipient feel observed or threatened. 
She felt a shiver down her spine when he leered at her in the dim alleyway.