Green’s Dictionary of Slang

smart, well-turned-out.

1786		Edinburgh Mag. Nov. 328/2: These wom,en were more men than our Jemmy-Jessamy countrymen.
1806–7		J. Beresford Miseries of Human Life (1826) 100: A Jemmy Jessamy lover [...] on one knee, in the dirt, before a coy May-pole Miss.
1844		Thackeray Barry Lyndon (1905) 168: This was very different language to that she had been in the habit of hearing from her Jemmy-Jessamy adorers.