A particularly pungent fart whose stench follows its creator wherever he or she goes for a certain period of time. The term is considered a "play on words" originating from Smart Missle.
"Oh my god, dude, it followed you! It's a smart fart!"
Smart fart: A person who knows they are about to let rip a very noisy fart will cough loudly so as to disguise the sound of the fart, quite often in rhythmical time to the blasts and, if very clever, will be in key too.
"We had no idea my uncle had farted as all we heard was his coughing, unfortunately his wry smile and the smell gave it away...so he admitted to his smart fart"