scale, n.² — Green’s Dictionary of Slang

scale n.²

[SE scale, on a reptile/fish; or ? a form of skin disease]

(US prison) a louse.

1918		H. Simon ‘Prison Dict.’ in AS VIII:3 (1933) 31/1: SCALE. Louse.
a.1968		‘Jimmie Tucker’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 66: She had scales on her cock, like a damned old sucker, / And her tits hung down like two tin buckets.

scale-backed ’un (n.)

a louse.

1923

J. Manchon Le Slang.

have scales (on one’s belly) (v.) [idea of being a snake or fish that crawls, i.e. crawl v.¹ (1)]

(Aus.) to be a sycophant.

1919		W.H. Downing Digger Dialects 43: scales (n.) — To have scales; to be a sycophant.
1922		Maryborough Chronicle (Qld) 27 Apr. 6/1: His abuse of anonymous writers having ‘scales on their bellies’, cuts no ice.
1924		(con. WWI) A.G. Pretty Gloss. of Sl. [...] in the A.I.F. 1921–1924 (rev. t/s) n.p.: scales. To have scales on ones [sic] belly; to be a sycophant; a crawler.