Since I get this quite a bit, might as well explain the story.
A long time ago in a state not so far away, a young man named Alex joined a group of folks known as GMSOG. As a new member, he needed to be given a new call sign/nickname, regardless of of previous history.
The rules were simple; it must be picked by the team, it must fit their personality, and must be some an animal.
Now, this young man, at the time, had just become a licensed aviation maintenance technician, performing regular maintenance and repairs to general aviation aircraft. Simultaneously during that time frame he visited some family that had immigrated to New Zealand for a multiple week adventure (cannot recommend it enough if you ever have that opportunity).
So start putting the two together you have a aircraft tech, a flightless bird if you will, and a highly recognizable national mascot and conveniently flightless, the kiwi bird.
As such that young man was forevermore known as Kiwi, joining the ranks of Tiger, Otter, Donkey, Platypus, Stag, many of whom remain friends to this day.