“A hot one in the eye, right off the bat. We indulge in this small base-ball remark, not to supply any lack of such remarks, but just for a ‘tip’ that we are ‘fly.’ We have some Spring Overcoats that are ‘fly’—fly front as well as very fly in appearance.” Indianapolis Journal, April 2, 1887, p1.
There are earlier uses of the term, but they are literal rather than metaphorical uses.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1888. The Davenport Morning Tribune (Dec. 2; Peter Morris) included an article about a conversation between a couple in which the husband complains bitterly about their son’s using too much slang. In the process the husband makes such comments as, “He is altogether too fly, that boy, and if I catch him with those boys again he’ll make a home run of the liveliest kind,” and “You are entirely off your base, my dear,” and “Let me hear that kid use slang again and I’ll give it to him right off the bat.”
The same article cited in the Dictionary appeared a few months earlier in the Evening Leader (Wilkes-Barre, PA) on July 11, 1888, 3, and in other papers between then and the December date of its appearance in the Davenport paper. But the clothing ad in the Indianapolis Journal in 1887 predates those uses.