The official language of one who speaks in a mumbled tone, usually someone too lazy to speak clearly or loudly enough for others to understand. Conversing with someone who speaks Mumblish usually leads to anger, frustration and blinding-rage.
Mumbler: "I asdfoindk to mlkasmdfo."
Non-Mumbler #1: "What the fuck did he say? I can't understand him!"
Non-Mumbler #2: "Fucking ignore him, he's speaking Mumblish again!"