shallow, n. — Green’s Dictionary of Slang

shallow n.

1. a hat [shape].

1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: Shallow. A whip hat, so called from the want of depth in the crown. Lilly shallow, a white Whip hat.
1811		Lex. Balatronicum [as cit. 1796].
1812		Vaux Vocab. of the Flash Lang.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 213: Shallow a hat; term acquired when the crown was worn shallow, and continued in the face of fact.
1826		‘Nocturnal Sports’ in Universal Songster II 180/1: [I] split my new shallow against the lamp-post.
1840		Flash Mirror 7: A modest-looking white or black shallow.

2. a basket, used by a costermonger [shape].

1837		Caledonian Mercury 5 Oct. 4/2: Biddy jumped in me shallow, and danced on me fruit.
1858		A. Mayhew Paved with Gold 75: Others have their ‘shallows’ fastened to their back with a strap, the holes at the bottom of some of the baskets having been darned with rope or string.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 27/1: Another mode of conveying the goods through the streets, is by baskets [...] the square and oval ‘shallow,’ fastened in front of the fruit-woman with a strap round the waist.
1870		Hotten Sl. Dict.
1876		C. Hindley Life and Adventures of a Cheap Jack 179: Sammy Schnoodle was a good old Israelite who used to travel with what is called a shallow. A shallow is a flat basket about three feet six inches long, two feet wide, with sides or edges rising four or six inches.

3. a costermonger’s cart.

1859		G.A. Sala Twice Around the Clock 33: There is a cobweb of wheeled vehicles of all sorts, from a cab to a hybrid construction something between a wheel-barrow and a costermonger's shallow.
1870		Dly Teleg. 2 June 2/3: A collision [...] between a costermonger’s shallow and a baronet’s barouche.
1880		Sheffield Indep. 27 May 7/3: The costermonger ‘shallow’ or van was full of happy faces.

4. a fool.

1902

H. Hill Caged xxvi n.p.: The local shallows thought this mode of entrance added dignity [F&H].

SE in slang uses

In phrases

shaking in the shallow (n.)

a game involving betting on the ‘heads or tails’ results of three coins shaken in a hat and tossed out onto a flat surface.

1860

R. Nicholson Rogue’s Progress (1966) 74: The game played then amongst the flash disciples of Dame Chance was ‘shaking in the shallow’ (tossing in a hat).