Green’s Dictionary of Slang

island n.

In phrases

see the island (v.) [the island is the inverted glass ‘hillock’ in the base of a wine bottle]

to drink to the bottom of a wine bottle.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Island, He drank out of the Bottle till he saw the Island; the Island is the Rising bottom of a Wine Bottle, which appears like an island in the center [sic], before the Bottle is quite Empty.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd edn).
1811		Lex. Balatronicum [as cit. 1788].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.