Green’s Dictionary of Slang

a man without genitals.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Rascal [...] a rascal originally meaning a lean shabby deer, at the time of changing his horns, penis, whence, in the vulgar acceptation, rascal is conceived to signify a man without genitals: the regular vulgar answer to this reproach, if uttered by a woman, is the offer of an ocular demonstration of the virility of the party so defamed. Some derive it from rascaglione, an Italian word signifying a man without testicles, or an eunuch.
c.1790		‘The Breeches’ Luke Caffrey’s Gost 8: Since the naked truth I must confess / This rascal kept his small cloaths on.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
[1859		Matsell Vocabulum 73: rascaglion A eunuch].
1980		Maledicta IV:2 (Winter) 194: A man without testes is referred to as a rascal (from the deer).