What does the expression "a retard of ninjas" or " a tard of ninjas" mean?
Does it have the same meaning as "a murder of crows" and "a litter of puppies"? Just a bunch of ninjas?

And I really would apreciate if you could tell me the origin of the expression.More info:
Some of my friends use it, so it might really be a joke of some kind. "Did you see that what he did to that tard of ninjas?"

Question

What does the expression "a retard of ninjas" or " a tard of ninjas" mean?
Does it have the same meaning as "a murder of crows" and "a litter of puppies"? Just a bunch of ninjas?

And I really would apreciate if you could tell me the origin of the expression.More info:
Some of my friends use it, so it might really be a joke of some kind. "Did you see that what he did to that tard of ninjas?"

Answer

"To retard" also means "to repel." If you retard an attack, you avoid it or stop it. We use the phrase "fire retardant" to describe material that cannot burn. I've never heard anyone say "a retard of ninjas" there needs to be more context, if you could post the whole sentence it would be helpful.

Answer

I'm not sure what the word is for a "group" of ninja, I'll trust chpr1 on the "hunk" thing...
what I would like to add is that it sounds like "a retard of ninjas" is an attack from someone, rather than a grammatical term. 
retard or tard mean fool:
you're such a retard = you're such a tard = you're such a fool = you're such an idiot ...etc.

So it sounds to me like a personal attack.

Answer

I think that the correct counter is a HUNK of ninjas.