Green’s Dictionary of Slang

a thief’s accomplice.

1860		in Times 5 Sept. [cited in Hotten].
1869		Australasian (Melbourne) 17 July 8/5: A [A] friend who watches while his ‘pals’ escape from prison is called a phillipe.
1873		Sl. Dict. 251: Philiper a thief’s accomplice, one who stands by and looks out for the police while the others commit a robbery, and who calls out ‘Philip!’ when any one approaches.
1881		Newcastle Courant 9 Sept. 6/5: Where were the thieves? Escaped; but how and when? Not before they arrived, or the ‘Philliper’ would also have gone.
1886		Graphic (London) 30 Jan. 23/1: Another name is ‘Philip,’ this being the signal word loudly given by a thief’s accomplice to announce the near approach of the police [...] The man who keeps a sharp look-out [...] while his ‘pals’ commit a robbery is hence called a ‘Philiper’.