Green’s Dictionary of Slang

(Irish) a fool.

1825		Pierce Egan’s Life in London 28 Aug. 243/2: [Irish speaker] ‘Molly, you were great with my brother afore you got the Omathaun that’s your husband now’.
1830		W. Carleton ‘The Hedge School’ in Traits and Stories of Irish Peasantry I (1868) 286: How can he, ye omadhawn, if we put a manwill in our pocket, and sware him?
1834		Belfast News Ltr 4 Feb. 4/2: Barney Bradley, you aumodhawn".
1837		‘Handy Andy’ in Bentley’s Misc. Jan. 28: Ride back for your life, you omadhaun!
1840		Vindicator 18 Apr. 4/1: ‘Bad scan to you, for a set of amadhons!’.
1848		Paul Pry 11 Dec. n.p.: Paul ishes to Know [...] If C—n, of Baker’s-row, was nursed by an Omadhaun; and if he commenced with nothing but his brogue.
1852		Kalida Venture (OH) 22 Oct. 1/3: You shall never take your grip off me, you omadawan!
1854		Bell’s Life in Sydney 15 July 2/6: Yer seed me not spoke nor bate the lying omadhaun.
1866		Wild Boys of London I 245: It’s a big lie, yer great omadhaun!
1876		A.B. Sedgwick Molly Moriarty 3: While I stand, hour by hour, like a great Ohmadaun as I am, gazing at her smiling face.
1879		H. Hartigan Stray Leaves (2nd ser.) 131: ‘’Tis little ye know of him, the poor omadawn’.
1883		M.L. O’Byrne Leixlip Castle 391: ‘I’m not an amadaun.’ ‘Put a’ hend to this tomfoolery!‘ cried Major Grub.
1885		Bulletin (Sydney) 31 Oct. 22/1: Like the Irishman who, when offended by his wife, fell upon an omadhaun in the kitchen and gave him the ‘grandfather av a batin’.
1893		F.P. Dunne in Schaaf Mr Dooley’s Chicago (1977) 227: We’re tired iv those big omadhons fr’m Mayo.
1895		Coburg Leader (Vic.) 6 Apr. 4/4: Sol, the Omadhaun, is going hunting in Grace Park.
1899		‘Pot’ & ‘Swears’ Scarlet City 506: I’ve flummoxed that ass of a Major, that pretentious omadhaun.
1900		Sydney Sportsman (Surry Hills, NSW) 28 Nov. 1/1: It proves them as not having sufficient cokum not to know when they are making first-class omadhauns of themselves.
1904		G.B. Shaw John Bull’s Other Island II ii: Oh, you’ve a dale to say for yourself, you butther-fingered omadhaun.
1906		C. M’Govern Sarjint Larry an’ Frinds 23: As annie omadawn of a cavalry man knows, de surprised horse lost his fire fer a full second.
1908		G. Seagram Bushman All 230: What you want, yer omadhaun is a polthogue on the head.
1910		P.W. Joyce Eng. As We Speak It In Ireland (1979) 210: Amadaun, a fool.
1914		Bulletin (Sydney) 24 Dec. 14/4: ’Twas you, you leather-headed omahdaun, who ilse?
1916		Nenagh Guardian 12 Feb. 3/1: Like an ould crack’d omaudaun, didn’t I go see the coursin’.
1929		B. Duffy Rocky Road 7: An ould omadaun like you.
1951		Irish Mthly LXXIX 492: God bless us and save us, what a terrible life his poor amadaun of a son had to put up with!
1953		A.J. Liebling Honest Rainmaker (1991) 87: [note] ‘Ahmaudon’ is third Avenue Irish for a graceless fellow, a schlemiel.
1964		W. Macken Scorching Wind 166: Amn’t I the stupid amadaun?
1979		J. McBride Traveller in the Glens n.p.: Now an ‘amadaun’ is a foolish person, and so is a ‘gulpin’, but the difference between the two is that the ‘gulpin’ is, in addition to being a bit of a fool, also conceited and self-assertive [BS].
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Amadáin (Omadhan) (n): idiot.
2006		V.H. Mayne Whom God Loves 76: He wished he was better informed on the subject of Home Rule and didn't sound such an amadaun.
2019		Twitter 17 Dec. 🌐 Thank you Mary for standing up to those racist omadons.