Learn English – Where does “Going out on a limb” come from

It was first used as an interjection in the 19th century: “They marched, and I amongst them, to face the enemy – heads up – step firm – thus it was – quick time – march!”

Then, at the beginning of the 20th century, it began to be used adjectivally, as in: “He was always right on the job, and looking ‘heads up’.”

Then, around the late 70s, it became a noun, probably through shortening of phrases like “heads-up alert” into “heads-up”: “It is regarded as being a heads-up on a sale.”

^{Source and references: the Grammarphobia blog}

The original telephone switchboards were literally patch boards: the caller and callee were physically connected with a wire (patch cable) to form a circuit.

Photo from here.