Green’s Dictionary of Slang

1. (US black/Aus.) a small room or space.

1868		‘Cong. Globe’ in DN IV:i 45: [Many of the National Banks] keep a little cubby of an office, loan no money, render no facilities, and yet draw interest on their circulation.
1884		(con. c.1840) ‘Mark Twain’ Huckleberry Finn 220: Up garret was a little cubby, with a pallet in it.
c.1895		F. Norris Vandover and the Brute (1914) 307: The cubby under the sink was abominably dirty.
1924		Lawrence & Skinner Boy in Bush 49: Whoever sleeps in this cubby. They has ter say their prayers, see?
1936		(con. 1830s–60s) ‘Miles Franklin’ All That Swagger 365: You have to thank old Wong that you had a cubby to retreat to.
1944		Cab Calloway New Hepsters Dict. in Calloway (1976) 254: cubby (n.): room, flat, home.
1951		P. Whelton Angels are Painted Fair 40: I passed [...] the bootblack cubby and the cigar stand.
1957		D. Niland Call Me When the Cross Turns Over (1958) 110: A sort of bach, you know. Everyone calls it the Cubby.
1960, 1975		Wentworth & Flexner DAS.
1981		P. Barton Bastards I Have Known 95: The ‘cubby’ as Jan called the secondary holding place, was an unglamorous narrow room.

2. (Aus., also cobby, cubbyhouse) a child’s playhouse, sited in the back garden.

1879		C.J. Kickham Knocknagow 504: Some children have what they call a ‘cobby’ under the hedge at the road-side.
1905		H.G. Wells Kipps (1952) 16: There was a corner under the ironing board which, by means of a shwl, could be made [...] a very decent cubby-house.
1959		G. Hamilton Summer Glare 18: Often we played in this piano-crate cubby house from daylight to dark.
2004		T. Winton ‘Small Mercies’ in Turning (2005) 91: He’s got a cubbyhouse.