Green’s Dictionary of Slang

(US) to communicate one’s intentions, usu. inadvertently.

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.