Green’s Dictionary of Slang

(orig. US) of a detective, to follow someone.

1869		N.Y. Times 19 May 5/1: As but little detective skill is required to ‘shadow’ an unsuspecting clerk for a few hours during the evening, unreliable persons are engaged for a small salary.
1871		Galaxy (N.Y.) Mar. 190: Two men from the same office are often detailed to ‘shadow,’ one the husband and the other the wife, and it occasionally happens that they have mastered the spirit of their calling so thoroughly that they do a little business on private account by ‘giving away’ each other.
1886		A.F. Pinkerton Dyke Darrel 28: A man had shadowed the detective since his departure from the railway office.
1892		H. Lawson ‘A Derry on a Cove’ in Roderick (1967–9) I 173: And when I try to find a job I’m shaddered by a trap.
1901		‘Lawrence Lynch’ High Stakes 222: It is not a shadowing expedition. It is a hold-up!
1913		H.A. Franck Zone Policeman 88 240: I shadowed a well-known American [...] in true dime-novel fashion.
1915		‘Doss Chiderdoss’ ‘The Best Examples’, Sporting Times 6 Feb. 1/2: We are shadowed in another sense [...] There’s a man there whom I noticed when we first together spoke!
1920		South Bend News-Times (IN) 6 June 19/1: We cannot shadow him as we would a criminal.
1931		Irwin Amer. Tramp and Und. Sl. 166: Shadow.–To follow or trail ; to observe.
1949		Monteleone Criminal Sl. (rev. edn).
1957		‘Ed Lacy’ Lead With Your Left (1958) 46: An all-day shadow job costs about seventy-five bucks per man.
1962		Ragen & Finston World’s Toughest Prison 817: shadow – To follow or trail to observe.