Sack refers to the male scrotum in this case. Synonym for "grow some balls" and other similar terms. Meaning: to accumulate or develop the courage or resolve to face a given situation.
"So you like her? Grow a sack and ask her out!"

"That's ALL you get paid!? Grow a sack and tell your boss you need a raise!!"
by Bleeorg December 23, 2003
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Toughen up, grow up, become more "manly"
Grow some sack, man. Pick out your own clothes and wear whatever the hell you want to!
by bcamero November 12, 2007
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A phrase spoken with an Irish or Scottish accent, meaning, be a man and get it done.
Guy1: I don't think i can do it.

Irishman: Why don't you grow a sack and do it, Laddie?
by Swagger-2094 July 21, 2011
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