A rule of thumbnail is a principle that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or determining if a Facebook profile is worth clicking on based on it's thumbnail.
Guy 1: Have you seen that chick's face?
Guy 2: No, i thought she wasn't worth looking at, according to my rule of thumbnail.
by starterel March 21, 2013

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