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!!"
"That's ALL you get paid!? Grow a sack and tell your boss you need a raise!!"
by Bleeorg December 22, 2003
Get the grow a sackmug. by bcamero December 7, 2007
Get the grow some sackmug. by Swagger-2094 July 21, 2011
Get the Why don't you grow a sack and do it, Laddie?mug.