Green’s Dictionary of Slang

(UK Und.) a mendicant villain who feigned dumbness, often claiming to have suffered at the hands of the infidel Turk who, on capturing him during a sea voyage, had torn out his tongue for denying Muhammad.

c.1566		Harman Caveat for Common Cursetours in Viles & Furnivall (1907) 57: These dommerars are leud and most subtyll people, [...] and wyll never speake, unless they have extreame punishment, but wyll gape, and with a maruelous force wyll hold downe their toungs doubled, groning for your charyty.
1608		Dekker Belman of London C2: In another troop are Gabling Domerers.
1612		Dekker ‘Of Dommerars’ O per se O M4: The Bell-man tooke his markes amisse in saying that a Dommerer is equall to the Cranke.
1622		Beaumont & Fletcher Beggar’s Bush II i: Higgen, your orator in this inter-regnum, / That whilom was your Dommerer.
1665		Head Eng. Rogue I 39: The rest in order thus [...] Dommerars, Glymmerers.
1674		Head Canting Academy (2nd edn) 57: Dommerars or Dummerars, are such who counterfeit themselves dumb, and have a notable art to rowl theuir tongues up into the roof of their mouth, that you would believe their tongues were cut out.
1688		R. Holme Academy of Armory Ch. iii item 68b: Give me leave to give you the names (as in their Canting Language they call themselves) of all (or most of such) as follow the Vagabond Trade, according to their Regiments or Divisions, as [...] Dommerars. [Ibid.] Dommerer, a Madman, a Bedlam.
c.1698		B.E. Dict. Canting Crew n.p.: Domerars, c. Rogues, pretending to have had their Tougus [sic] cut out, or to be born dumb and deaf.
1707		‘Rum-Mort’s Praise of Her Faithless Maunder’ in Farmer Musa Pedestris (1896) 36: Crank and dommerar thou couldst play, / Or rum-maunder in one day.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Dommerer, a beggar, pretending that his tongue has been cut out by the Algerines, or cruel, and blood thirsty Turks, or else that he was born deaf and dumb. (cant).
1811		Lex. Balatronicum.
1812–82		B.M. Carew Life and Adventures.
1849		Examiner 14 Apr. 10/2: Besides counterfeiting the falling sickness the Dommerat, or Dommerer, had a trick of doubling his tongue in his mouth and ‘making a horrid or strange noise’.
1859		Matsell Vocabulum.
1861		(con. 15C) C. Reade Cloister and Hearth (1864) II 33: Come with me to the ‘rotboss’ there, and I’ll show thee all our folk and their lays [...] ‘Rufflers,’ whipjalks,’[sic] ‘dommerars,’ [...] ‘autem morts,’ and ‘walking morts’.