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1.
in music, when the song slows down. but it can be used to call some one a retard or slow.
you're such a ritardando!
by nerm November 19, 2003
 
2.
It is actually a music term but it is also popularily used as a sustitute for retarded.
Ewww, We have to write an essay tomorrow? That's ritardando!
by Paula E. March 02, 2006