"nz", or "NZ", or even "November Zulu", is a combination of the first and last letter of my last name.
Several years ago, during my military service, I have worked in air
traffic control. There, it was crucial to know who did what,
and therefore everything written on paper (or cardboard strips) was to be
"initialed", that is signed with initials. Even in telephone or radio
communications, both sides would acknowledge understanding of the passed
message by giving their initials to the other side, which then would write
that down.
In the ATC tower I worked in, these initials were created by combining the
first and last character of one's last name. So mine became "NZ".
Since I was so drilled to using "November-Zulu" for just about everything and on everything, I stuck to it, and am using it for several logins, my primary mail address, my car registration plate, and other things today.
I do not use it on civil telephone calls, though.