1. To put or pack something into a sack or bag. In this usage, a noun or pronoun can be used between "sack" and "up." My first job was sacking up people's groceries at the store down the road for $5 an hour. Don't worry about wrinkling the clothes. Just sack them up.

2. vulgar slang To start acting in a strong, confident, and/or courageous manner, especially after having previously failed to do so. In this usage, "sack" is slang for the scrotum, a reference to testicles, which are used figuratively to represent confidence, courage, bravado, etc. The phrase is not exclusively applied to males. Often used as an imperative. You need to sack up and ask your boss for a raise already! What's he going to do, fire you? Janet, I know you're nervous about asking Tom out on a date, but just sack up and give it a shot!

3. slang To have sexual relations with someone. In this usage, "sack" is slang for a bed. A: "I heard that Tom has been sacking up with some 30-year-old accountant from his old law firm." B: "Wow, he hasn't even been divorced a month!" I can't believe you and your ex-boyfriend sacked up again.