Green’s Dictionary of Slang

1. (US) exact, fit, stylish, fashionable.

1861		Vanity Fair (N.Y.) 28 Sept. 153: Next we shall see the Keynote alluded to as the ‘jimmy’ note, while the occasional sharp will be designated as a ‘teerer,’ and the flat as a ‘muff’.
1899		Ade Fables in Sl. (1902) 129: The Kind that wears a Cutaway, with a White Flower, in the Morning [...] and a jimmy little Tuxedo at Night.
1922		Beloit Dly Call (KS) 14 Sept. 8/4: ‘Some of your contemporaries I’ve seen “slammocking” about [...] but you are always — “jimmy”’.

2. see jemmy adj.²