The phrase "sack up" is an informal term that is often used to describe someone being brave or taking responsibility for their actions. Some other synonyms that could be used in its place include: man up, toughen up, pull up one's boots, grow a backbone, stand tall, be courageous, show grit, or be resilient. All of these expressions indicate that a person should gather their courage and face a challenge head-on. They can be used in many different contexts, from sports to business to personal relationships. Choosing the right synonym depends on the situation and the desired tone.