"fly" = "hip" (or, if you prefer, "hep")

[George Thompson]

Under "fly", adjective, HDAS discerns two related senses: sense 1a (=
wide awake) *1724, *1821, *1850, &c. (all English); 1872 (US); sense 1b
(= aware) *1811, *1812 (both English); 1839 (US)  (I abbreviate the
definitions and leave out the supporting quotations, but we all sleep
with HDAS on the nightstand, don't we?)

Whichever sense the first of these two below goes under, it is a pretty
neat antedating in the U. S.  The fact that it comes 50+ years before
the 2nd earliest English citation is all right, too.

1749:   That the fly Ones should not suspect you for a Courtier, you
have been likewise very arch in giving us to understand, that you had
been heretofore pleased to encourage and support the Party.
        N-Y Gazette Revived, January 16, 1748-9, p. 1, col. 1  [from a
political diatribe, responding to last week's political diatribe]

1824:   The carriages, wagons, horsemen, and pedestrians, who seemed to
make Coney Island their place of destination, amounted to a
considerable number; the roads were lined and adorned with them in
every direction; the regulars were numerous -- the knowing ones were
up, and the downy ones were all fly.
The Emerald, October 16, 1824, p. 109, cols. 1-2.  [a report on a
prizefight]

GAT

George A. Thompson
Author of A Documentary History of "The African Theatre", Northwestern
Univ. Pr., 1998, but nothing much lately.