Green’s Dictionary of Slang

1. (UK prison) prison uniform.

1838		Dickens Oliver Twist (1966) 184: His wardrobe was, in truth, rather out of repair [...] in consequence of having worn the regimentals for six weeks past.
1889		Freeman’s Jrnl 18 Jan. 5/3: A convict in full prison regimentals was brought down to court.

2. (US police) police uniform.

1931

(con. 1900s) C.W. Willemse Behind The Green Lights 103: Tim was patrolling his beat in his proper regimentals.

3. any uniform.

1941

J.M. Cain Mildred Pierce (1985) 526: [of a barman] Her temper didn’t improve when Archie took off his regimentals at nine sharp.