A fork of Rural Dictionary
Tonto was the sidekick of the Lone Ranger in the old westerns.
The Lone Ranger and Tonto are walking in a meadow when Tonto gets off of his horse, leans down and puts his head to the ground. When he comes back up he says, "Whoa....many buffalo come." and the Lone Ranger asks, "How do you know?" Tonto replies, "Ear soaking wet."
When someone completely loses the plot and goes mental at you for no reason. The word can be used in both funny and serious situations. Teachers, partners and mothers are most likely to be described as going "tonto".
Person 1: "You coming out tonight?" Person 2: "Sorry can't my mum's gone absolutely tonto for no reason and she's lost the plot."