It should be noted in context that original the 747 could, like the 737 before it, service airfields too small to have service centers. Therefore, it was important that another 747 could ferry in all parts needed to service a potentially damaged 747 at these locations, including an entire engine.
Most aircraft with such capability are identified by the inclusion of built-in stairs for passenger ingress/egress, which the 747-100 had.
I am dating myself but I'm old enough to have watched the Simpsons episode where this word was invented in its first airing. Maybe it is a word now, I mean it's no more or less ridiculous than other words, which somebody just made up at some point too.
Most aircraft with such capability are identified by the inclusion of built-in stairs for passenger ingress/egress, which the 747-100 had.