tap [OE] A tap was originally a stopper for a cask. It controlled the flow of liquid, so the same word came to be used for the fitting which controlled the flow of water elsewhere. Drink from a cask that was ready for immediate consumption was on tap. From the 1860s tap began to be used in reference to listening in secretly to a telegraph and then a telephone, from the idea of ‘siphoning off’ information. Tap [ME] in the sense of ‘strike lightly’ is a completely different word, which probably represents an imitation of the action in its sound.