The phrase "sack up" is an informal expression that can mean to gather courage or to prepare oneself for a difficult situation. It is often used to encourage someone to be brave or to take action despite fear or uncertainty.
In a more advanced context, "sack up" can be used to encourage someone to take responsibility for their actions or to make a decision. - "If you want to succeed in this project, you really need to sack up and take charge."
While "sack up" primarily means to gather courage, it can also be used in a more literal sense to mean to pack or store things in a sack or bag, though this usage is less common.
"Sack up" is a motivational phrase used to encourage bravery and action.