not understandable (adjective) unimaginable: mind-boggling
Sinonime: bat of the eye,
breath,
crack,
flash, jiffy,
juncture, minute, nothing flat,
occasion,
point, sec,
second, shake*, short while, split second, tick,
time, trice, twinkling,
while, wink
Antonime: believable, describable, imaginable