Green’s Dictionary of Slang

1. a casual pauper.

1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 202: Casuals — twopenny lodgers for the night in the rookeries.
1865		T. Archer Pauper, Thief and Convict 47: At a pretty early hour the ‘casual’ is aroused, and receives another slab of bread by way of breakfast. It often happens, however, that neither breakfast nor supper is eaten, the needs of the tramps extending not so much to food as to rest.
1872		Galaxy (N.Y.) Mar. 317: Another class of casuals are the young men who are suddenly turned out of their boarding-houses for non-payment.
1892		Lloyd's Wkly Newspaper 3 Jan. 6/4: There was something irresitably comical about the cut of this amateur casual.
1906		O.C. Malvery Soul Market 58: The dreaded ‘inspector’ [...] looked at every ‘casual’ closely.
1910		Derbys. Advertiser 2 Dec. 25/4: A Night in Derby Casual Ward [...] By Amateur Casual.
1933		Western Morn. News 27 Feb. 3/6: The Master [i.e. of the Workhouse] said 81 men were admitted during the previous week-end. [...] More casuals were visiting the institution than could be accomodated.

2. the casual ward in a hospital.

1850		Morn. Chron. (London) 22 Jan. 5/6: There’s far more good people in the straw-yards than the casuals — the dodgers is less frequent there, considering the numbers.
1867		Cheltenham Chron. 3 Dec. 2/1: Come, you mustn’t sleep here [...] why don’t you go to the Casual, — that’s for fellows like you.
1887		Pall Mall Gazette 24 Oct. 1/2: The accommodation in casuals and workhouses. [...] This ward [...] holds ninety about the usual number for a London casual.

3. a part-time labourer or other employee.

1878		Belgravia 34 225: There are four classes of people who knock at this door. The family, tradespeople, visitors and casuals [not to mention run-away knocks] .
1889		C. Booth Life and Labour I. 202: The work of the casuals was a dead loss to the contractor.
1930		L.G.D. Acland Early Canterbury Runs (1951) 111: He did not of course pay the casuals very big wages.
1936		‘Josphine Tey’ Shilling for Candles 84: The coat [...] had been taken by a ‘casual’.
1965		F.J. Hardy Yarns of Billy Borker 115: He was a casual wharfie at the time I’m telling you about, during the Second World War it was, and they call casuals ‘seagulls’.

4. (N.Z. prison/und.) a detective.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 38/1: casual n. a detective.

5. (US campus) an inadequate.

2014

C. Eble (ed.) UNC-CH Campus Sl. Spring 2014 Fall 2: CASUAL — person lacking expertise or experience: ‘He claims to be a basketball fan, but he can’t even tell a technical foul—what a casual’.