Rude nickname given to Albus Potter by fellow-First Year Karl Jenkins during their first flying lesson when Potter’s broomstick refused to rise from the ground when ordered “Up!” (CC1.4). Karl was implying that Albus did not have magical ability, like a Squib.
The scene with Albus Potter struggling to control a broomstick is reminiscent of problems Hermione and Neville had with broomsticks during their first Flying Lesson in 1991: "Hermione Granger's had simply rolled over on the ground, and Neville's hadn't moved at all. Perhaps brooms, like horses, could tell when you were afraid, thought Harry..." (PS9). However, since both Hermione and Neville were successful with other kinds of magic, the inability to make a broomstick fly clearly had nothing to do with their potential. It just wasn't one of their gifts as flying was to Harry or Draco.