Green’s Dictionary of Slang

1. the frenum or ligament of the penis [it is attached to the ‘head’ of the penis].

1650		J. Bulwer Anthropometamorphosis 213: [The] Prepuce is tyed in the lower part with a membranous band [in Latin] frenum, in English the Bridle.
1654		Mercurius Fumigosus 16 13–20 Sept. 142: [A couple were found,] he with his hand upon her nunquam satis, and she holding his Nagg by the bridle.
a.1749		Robertson of Struan ‘Marriage Song’ Poems (1752) 96: Push till the Muggles [i.e. orgasm] seize the Fair And the unruly breaks his Bridle.
1749		Hist. of the Human Heart 44: Camillo [...] pushing boldly to pierce the obstructing Hymen, broke the Bridle of his fiery Courser, and was obliged by the extreme Anguish to retreat from the Field of Battle.
1813		in Order of the Beggar's Benison and Merryland (1892) 35: [The] Cord — the Bridle or Frœnum — from its reining in or checking in some measure the full extension of the penis when erected.

2. (UK und.) a hat.

1935

Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 2: Bridle: hat.