cu
"Call Unix" — the UUCP suite's terminal program, which dialled another computer and turned your terminal into one of its own.
Type cu followed by a phone number or system name, the modem dialled, and everything you typed went straight to the other machine. It shipped with Unix from Version 7 in 1979, and BSD got its own counterpart in tip. Commands for cu itself started with a tilde at the beginning of a line: ~. hung up, while ~%take and ~%put moved text files with no error checking whatsoever.
The tilde convention outlived it. rlogin adopted it, and SSH inherited it from there — which is why ~. still kills a frozen SSH session. cu itself is mostly used today to talk to serial consoles on routers and single-board computers, where nobody has come up with anything simpler.