Green’s Dictionary of Slang

(UK Und.) to look at closely; to spy on.

1665		Head Eng. Rogue I 52: The Cul Snylches, The Man eyes you.
1674		Head Canting Academy (2nd edn).
c.1698		B.E. Dict. Canting Crew n.p.: Snilch, to Eye or See any Body. The Cull Snilches, c. the man Eyes you or Sees you.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
c.1850		Duncombe New and Improved Flash Dict.