A hundred pound weakling aspires fruitlessly to affections of the prettiest girl in town. She see him combing his hair while looking in a mirror, and makes fun of him.
""'Hello, Willie!' says Myra. 'What are you doing to yourself in the glass?'

"I'm trying to look fly,' says Willie.

"'Well, you never could be fly,' says Myra, with her special laugh, which was the provokingest sound I ever heard except the rattle of an empty canteen against my saddle-horn."

What does "fly" mean in this context?