Green’s Dictionary of Slang

to make contact; to communicate one’s intentions, usu. inadvertently.

1828		T. Creevey letter 8 Mar. in Gore Creevey’s Life and Times (1934) 262: [H]appening to catch his eye, I telegraphed him, and having instantly abandoned his stirrup, he called out, ‘I’ll come over to you’.
1923		H.C. Witwer Fighting Blood 72: You got a heart and you can sock! [...] but you telegraph the other guy everything you’re going to do.
1934		V.G. Burns Female Convict (1960) 29: She would telegraph to Frenchy the exact cards held by her opponents.
1945		E. Nichols Hunky Johnny 68: Christ, he telegraphs every curve he throws [DA].
1950		L. A. Times 7 May [comics] 13: He doesn’t throw that right fast enough. He telegraphs it [DA].
1961		J. Scarne Complete Guide to Gambling.
2000		D. Benioff 25th Hour 9: Monty assesses the ballers quickly, accurately, and disdainfully. [T]he big man down low telegraphs his every shot.