sack = ?

Already Beaver cannot understand where they found the sack to intervene in the first place, and later the others will say the same.
Judging only by the way it's used, I'd say it's what we sometimes call "nerve" or "gall," or in a dangerous endeavor, "brass balls."

(It's none of your business! What gives you the right to stick your big nose in?) But I've never before heard "sack" used in that way.

Edit. Yeah, I was close. Urban Dictionary gives "sack up" as getting up the courage for a daunting task.

Sometimes people intervene in the business of others because they're "nosey," but in this case it's because they're courageous.
Sack is slang for testicles. So not a very polite way of expressing this.
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