From LQWiki

ENQ is an abbreviation for "enquire"; it is assigned as a control-code in the US ASCII character set. This assignment comes down to us from the days of Teletype machines (keyboards and rudimentary display devices which were interconnected without computers, so the operators needed ways to communicate their intentions outside the normal stream of printed characters).

ENQ is the control code used by a sending Teletype station to query a target station, asking if it is available to receive a message. The correct control-code responses are ACK or NAK, depending on the availability of the target station, in the opinion of its operator.