Green’s Dictionary of Slang

telegraph v.

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

[US]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.
[US]V.G. Burns Female Convict (1960) 29: She would telegraph to Frenchy the exact cards held by her opponents.
[US]E. Nichols Hunky Johnny 68: Christ, he telegraphs every curve he throws [DA].
[US]L. A. Times 7 May [comics] 13: He doesn’t throw that right fast enough. He telegraphs it [DA].
[US]J. Scarne Complete Guide to Gambling.