Green’s Dictionary of Slang

extensive adj.

showy, vulgar.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 136: extensive frequently applied in a Slang sense to a person’s appearance or talk; ‘rather extensive that!’ intimating that the person alluded to is shewing off, or ‘cutting it fat’.
1864, 1867, 1870		Hotten Sl. Dict.
1903		Truth (London) 18 June 1678/3: Slang terms: [...] cheese, clipper, crack, crushing, down-the-road, extensive [etc] .