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1.
Worse than retarded. Retarded to the tenth power. retarded
"Mane, that nigga got 30 inch spinners on a Geo Metro. Mane, that nigga retonto."
by Cha-Chee Brazil December 12, 2006
 
2.
A term originating in South Louisiana, and primarily used in such (i.e. by Baton Rouge rappers such as Lil Phat and Lil Boosie).

Retonto is a disambiguation of 'retarded' in a context meaning 'dumb', 'stupid', 'crazy', etc. It originates as a cross-language compound word consisting of the English word 'Retarded' and the word 'Tonto' which, in Spanish and Italian means 'Dumb' or 'Silly', and in some Native American languages means 'wild one'.

Therefore, Retonto is essentially a cross between 'Retarded', 'Dumb', and 'Wild'.
"Them boys don't go HAM, they go straight Retonto"

"Baton Rouge is straight Retonto"
by BR Phat June 13, 2013