You won't find it online.

How odd. You'd think that, in this day and age, it would have made it into
an online reference by now.

Yeah...look up "crack gag." Another common studio term you won't read anywhere.
It's an extension of "gag light" and came into use following popular use of
slit scan photography in the late 60's and 70's.
Wally. You know less than nothing. Less than less than nothing in fact!

RB