On Newgrounds - to rate a flash or audio piece 0/5 for the sole purpose of bringing down the rating of that piece of work, for the sole purpose of annoying the author.
Elsewhere - to rate a piece of work the lowest possible grade for the same purpose. (Also called 1-bomb where the lowest rating is 1, but not commonly used outside of Newgrounds.)
Considered a form of spam.
<gnuwtey> Aw man, my flash's rating went from 4.04 to 2.85 with only 6 votes. Stupid zero-bombers.
<dernston> I agree, those zero-bombers really are annoying.
11👍 1👎