Sometimes after a long evening The Waldos would end up at Jack in the Box. As everybody knows, you drive up to the speaker microphone and give your order.
One time, the Waldos ordered a Jumbo Jack. The order taker/cashier would normally turn on the speaker-microphone and take your order, and then turn off the speaker-microphone.
This one time, the Hispanic-accented order taker left the speaker-microphone on and The Waldos could hear him yelling back to the cook, “Yummo Yack”. The cook, who also could not easily say Jumbo Jack, could be heard yelling back the order confirmation, “Yummo Yack”.
The Waldos would thereafter say “Yummo Yack” any time for no particular reason other than it is quite fun to say. (You. Reading this paragraph right now. Go ahead and say out loud, “Yummo Yack!” You cannot say it out loud without a smile on your brain or face. It may be one of the catchiest of The Waldo catchphrases.)