Green’s Dictionary of Slang

splendid, esp. as regards one’s dress.

c.1880		F.McGlennon [perf. Kate Harvey] ‘You Fancy Yourselves, Do You!’ You lay your snares, put on airs, / Try to look ‘immensikoff’.
1887		Bessie Bellwood ‘What-Cheer ’Ria’ 🎵 Oh Ria she’s a toff and she looks immensikoff.