Top Definition
Literally short-hand for "Small Planet. Be Nice."
(#SPBN on twitter courtesy @ennuizle )
Text or Twitter short-hand for "Small Planet. Be Nice." and usually accompanied by a judgmental emoji (often • - • ) that shows the receiving person that you "saw what they did..."

Often used as a 'Tsk'-ing or punitive phrase with someone familiar who just got snagged/called-out doing/saying something mean or naughty or otherwise receiving of stern judgment from their matriarch.

SPBN is often used in conjunction with a 'debating' classroom setting to tell someone secretively that they've gone too far in their argument and to back off or down.
by eric Hews August 15, 2014

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