The state of a telephone line that allows dialing and transmission but prohibits incoming calls from being answered. You are off-hook when you remove the handset from the base unit of a stationary phone or press Talk on a portable phone.
The term stems from the days when the handset was lifted off an actual hook. When the handset was removed, a spring caused contacts to press together, closing the circuit from the telephone to the switchboard in the central office (CO). When the handset is placed back on the base, it is said to be "on-hook" and can receive an incoming call.
There Really Was a Hook
The hook on this Kellogg telephone (circa 1899) was attached to a spring. When the handset was removed, the contacts inside completed the circuit between the telephone and the central office.