Goodly Lord, what a wit-snapper are you! then bid them
Launcelot:The Merchant Of Venice Act 3, scene 5, 49–52
That is done too, sir, only "cover" is the word.
Come sir, are you ready for death?
Over-roasted rather: ready long ago.
Jailer:Cymbeline Act 5, scene 4. 151–154
Hanging is the word, sir. If you be ready for that, you are well
Shakespeare uses some form of "___ is the word" at least five times in his plays, and I've presented here the earliest and one of the latest occurrences. For the clown Launcelot, who's been put in charge of supervising a banquet, "cover" ("lay the tablecloths") is the word; for King Cymbeline's jailer, "hanging" is the word. Launcelot means something like "I'm only waiting for the command 'cover' "; the jailer uses the phrase in a sense slightly closer to ours. He too waits upon a command—"hang 'im"—but he also means "hanging is the business at hand; the word 'hanging' is the precise word for your situation."
That Shakespeare used this expression with some frequency has led certain scholars to assume that it's proverbial. On the other hand, Shakespeare may have invented it and then grown paternally fond of it. No one has yet turned up an earlier example in surviving writings.