Green’s Dictionary of Slang

an inhabitant or native of the county of Middlesex.

1662		Fuller Worthies (1840) II 313: ‘A Middlesex clown’ Some English words, innocent and inoffensive in their primitive notion, are bowed by custom to a disgraceful sense.
1790		Grose Provincial Gloss. 84/1: Fuller and Ray suppose the Middlesex yeomen to have been styled clowns, from their not paying the same deference to the nobility and gentry, that was shewn by the inhabitants of more remote counties, to whom the sight of them was less common. Perhaps it was likewise owing to the sudden contrast between the behaviour of the inhabitants of the metropolis, and of some of the small villages a few miles off; several of which [...] are more countrified than the rustics. of Cornwall or Northumberland.
1856		Era (London) 2 Mar. 9/3: He is not acquainted with the old proverb, ‘f all the clowns of England a Middlesex clown is the worst’.
1878		Luton Times 12 July 7/5: Middlesex — ‘A Middlesex clown’.