When a grown man decides to make sensual romantic mastabatory love to his own penis in the confines of his dorm room/MAF SRT room. This "party" usually occurs while the lone man is vigorously crying and listening to Mariah Carey's new smash hit "Touch My Body."
"Hey Woodward, you wanna come out to the club and pick up chicks with us tonight?"
"No man, I think I'm gonna drink a protein shake and have a one man party."
When a man, finding himself home alone on a weeknight with nothing to do, decides to have a drink and ends up getting shit-faced over the course of several hours.
Steve walks into work rubbing his head. "Oh man, I have a terrible hangover." "But it's a Tuesday!" Bob exclaims. "Were you out on a date?" "No, I had a one-man party last night. I watched some Netflix, played some guitar, drank a lot of whiskey."