Green’s Dictionary of Slang

1. a derog. term for a German or Dutchman [supposes an image of the German or Dutchman as all display but no substance and as races that, for all their external show, can be ‘eaten up’ by an Englishman].

1630

J. Taylor ‘A Brood of Cormorants’ in Works (1869) III 9: As many of the Country lordships slips / Flapdragon like, by his insatiate lips.

2. venereal disease [the ‘heat’ affects the penis].

c.1698		B.E. Dict. Canting Crew n.p.: Flapdragon a Clap or Pox.
1700		N. Ward Rambling Rakes 8: All those numerous Intreaguers that resort there, tempting enough to break a Commandment with, or run the Risque of a Flap-Dragon.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.