Green’s Dictionary of Slang

Hampstead Heath sailor n.

[apart from the Round Pond and a number of bathing ponds, the Heath is dry land]

a very poor sailor, no sailor at all.

1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues.