Green’s Dictionary of Slang

1. (Irish) an agarian Protestant gang, predecessors and rivals of the Orange Order.

1777		Leeds Intelligencer 6 May 2/1: We learn from Balinrobe in the county of mayo, that the Gaol of that town was broke open [...] and every person therein set at liberty, by a gang of [...] Peep-of-day boys.
1798		Gaz. of United Sates (Phila., PA) 26 Nov. 3/2: When I beho’d a man evidently plotting y destruction, it is of little import whether he strike under the name of the highwayman [...] or a peep o’ day boy, an assassin.
1798		‘Little Peru’ in Hilaria 53: My tight peep of day boys, leave stones, bricks and mortar / Come one after t’other, rise all in a mass.
1800		Gaz. of United States (Phila., PA) 25 July 3/1: The gentleman it seems was Peep-o’day Boy in Ireland.
1887		(ref. to 1798) Wheeling Dly Intelligencer (VA) 16 Dec. 1/7: The play takes its name from the principal character [...] a leader of a band of ‘peep o’ day boys’ of 1798.

2. (UK Und., also break of day boys) one who stays up all night carousing; cite 1851 appears to refer to a specific Sydney gang.

1821		Egan Life in London (1869) 311: The glass was pushed about so quickly, that the ‘First of the Month’ was soon forgotten, and we kept it up till very long after the regulars had been tucked up in their dabs, and only the Roosters and the ‘Peep-o’-Day-Boys’ were out on the prowl for a spree.
1825		Egan Life of an Actor 256: comical dick was a peep o’ day companion, who never left his glass or hids friend till somnus closed his eyes.
1827		Egan Anecdotes of the Turf, the Chase etc. 192: Like all the ‘peep-o’-day boys,’ nothing is yet the matter with his constitution.
1832		Ely’s Hawk & Buzzard (NY) Sept. 15 n.p.: A parcel of low, dirty, break of day boys .
1834		‘Jack Downing’ Andrew Jackson 5: Battels atween the White boys, harts of Oke, harts of Steel, peep of day boys [etc.] .
1839		Morn. Herald (NY) 10 June 2/5: ‘Peep o’day Boys’ — The prevalence of wet weather [...] acted as a damper on the sports of the rowdies.
1841		Era (London) 21 Nov. 2/2: You thieving peep-o’-day spalpeens [...] I’ll clean your dirty muzzles with the fist I have in my hand!
1843		R.S. Surtees Handley Cross (1854) 66: Doleful must be a trump [...] Keen fellow too — Peep-of-day-boy.
1843		Bellew Memoirs of a Griffin II 144: Others [...] dropped off to bed, though abused by the peep-o’-day boys for our recreant qualities.
1850		Fast Man 7:1 n.p.: A peep o’ day boy is mistaken—very much mistaken.
1851		Bell’s Life in Sydney 15 Nov. 3/1: [headline] Peep o’ Day Boys [...] three of the well-known peep-o’-day-boy mob.
1865		Eve. Star (Wash., DC) 8 Apr. 1/2: First time of the Roaring irish Farce [...] The Peep o’ Day Boys Phelim O’Flannigan, a Peep o’ Day Boy. Biddy Muldoody, a Peep o’ Day Woamn.