After learning how to program the PDP-7, Thompson, Ritchie, Ossanna, and Canaday began to program the operating system that was designed earlier. After writing the file system and a set of basic utilities, they wrote a PDP-7 assembler so they could program directly on the PDP. By 1970, the basic elements of the operating system were in place, but since it could only support one user, Brian Kernighan jokingly named it the Uniplexed Information and Computing System (UNICS) as a pun on MULTICS. When multiprocessing functionality was added a short time later, the name was changed to "Unix", which is now just a name and not an acronym for anything. (
https://www.livinginternet.com/i/iw_unix_dev.htm)