Short for Lamborghini Murcielago. Also notice the way he breaks up the word to throw an extra “mercy” drop in there.
It’s pretty unlikely, but the way Pusha says Murci-Lago could be a slur of largo, which in musical notation is a very slow tempo. The opposite of largo is the fastest tempo or pronto, as he references later in the same line.
To help improve the meaning of these lyrics, visit "Mercy" by Kanye West (Ft. 2 Chainz, Big Sean & Pusha T) Lyrics and leave a comment on the lyrics box