People call it a jab because some of these use a really short needle that is more of a jab than a shot.
Vaccines are administered in different ways. Some are oral.
Many use jab in place of vaccine because this covid “vaccine” is not really a true vaccine.
No, I think folks call it that either because they've
seen other people doing so and want to be "cool"...
or otherwise because they are gay and likely diseased.