Green’s Dictionary of Slang

an iron fetter worn on one leg only.

1591		Greene Art of Conny-Catching in Grosart (1881–3) II 31: Clap a strong paire of bolts on his heeles, and a basill of 28. pound weight.
1724		Hist. of the remarkable Life of John Sheppard 8: He and his MATE were now in [...] Prison, himself loaded with a pair of double Links and Basils.
1755		Memoirs of Capt. P. Drake I 106: One of my Irons taken off [...] only one of the Bassils, which did not weigh above two Pounds out of thirty .
1865		G.A. Sala in N&Q Ser. 3 VIII 369/2: The iron ring or fetter which English convicts were wont to wear round one ankle was called a Basil.
1890		Bulletin (Sydney) 30 Aug. 24/2: A slow pace meant that at least twenty steps could be taken without a fresh piece of skin being worn off by the basil.