Green’s Dictionary of Slang

tommy! excl.

In phrases

tip tommy (v.) (also cry tommy)

to keep a lookout, thus n. tommy, a lookout.

1721		Proc. Old Bailey 11 Oct. 🌐 The Prisoner and himself went to the Window whilst Buxon stood at some distance, and if any Body came he was to give notice by crying Tommy.
1729		Life of Thomas Neaves 16: Neaves stop’d the Chair with a cock’d Pistol in his Hand [...] whlst his Accomplices were only Tomys, that is, plac’d at a certain distance , to give timely Notice if any Persons approach’d.
1732		Proc. Old Bailey 22 May 🌐 [...] and likewise to give him the Word Tommy, which is for him to understand, that some Body is coming.
1733		Proc. Old Bailey 28 June 🌐 As we had agreed, he cry’d, Tommy, when the Coast was clear; upon hearing the Watch word, I took the Cloth on my Shoulder, and went with it to Mrs. Wincox's.
1739		Proc. Old Bailey 21 Feb. 🌐 Leonard was to stand on the other Side of the Way, and to cry Tommy! Tommy! when any body came by: And he calling out Tommy, as soon as we had got the Man's Hat and Wig and Gun, we had not Time to rifle his Pockets, but were obliged to run away.