Fzfifteen

Fzfifteen is equal to \(15^{15}\) = \(15\uparrow\uparrow2\) = 437,893,890,380,859,375

Etymology
Fzfifteen is made out of the Fz- prefix and fifteen, which ends up becoming \(15^{15}\) when concatenated backwards, ergo, 13 to the power of itself.