Definition from Wiktionary, the free dictionary
Etymology 1 
- To make a wry face in contempt, or to grin in scorn; to deride; to sneer; to mock; to gibe.
- To grin with an air of civility; to leer.
- Grinning and fleering as though they went to a bear baiting.
to make a wry face in contempt, or to grin in scorn
Etymology 2 
fleer (plural fleers)