An abbreviation of a right-extremist copypasta which stands for Total (Racial slur which I cannot say) Death.
It originally started on 4chan, but was later satirised into irony by soyjaks.party and other soyjak forums.
The copypasta itself, which is better known as the "kill behead roundhouse" copypasta, starts of with "Kill N, Behead N, Roundhouse kick a N into the concrete" and the copypasta continues. For more information: search up tnd copypasta.
"TND will occur within 72 minutes"
A quote by SoyTube