"Córtalas" is an expression associated to children when they want to break up friendship because they're mad at their friends.
"Canicas" are marbles, and children of former times used to play a lot with these toys. As those games frequently needed a child lending marbles to a friend, once friendship was broken, toys had to be given back to their original owner.
"Córtalas y regrésame mis canicas" is a joking way to say that you're mad at a friend and you're breaking up friendship, so you want your toys back, as if you were children fighting.
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