I am reading the script of a TV series "How I Met Your Mother"
The script has this sentence:
Hey, you want to do somethin' tonight? Okay, meet me at the bar in 15 minutes. And suit up! Where's your suit? Just once, when I say suit up, I wish you'd put on a suit.
What does "suit up" means please?
I tried to use a dictionary but it seems it is not a formal phrase.