look up any word, like fleek:
 
1.
A slang term for saying down syndrome, usually used to insult someone. Derived from the fact that down syndrome is caused by having 3 21st chromosomes. 3 *21=63
Example 1- Wow look at Travis there, he's totally acting like a sixty-three

Example 2- Hey man don't make fun of Zach, he's a sixty-three. That's why he fails so much.

Example 3-
Person 1: Hey look at those kids, what are they doing?
Person 2: I remember when I was a sixty-three
by >tdawgunit May 04, 2011