When someone does something that isn’t very Plus Ultra of them to do. Like insulting someone else, or the Japanesename in general. Relates back to the anime “My Hero Academia”.
Susie: Bethany looks really weird!
Me: Susie, that’s not very Plus Ultra of you!
Used as a response to point out an unintentional double-entendre in which the original speaker could be taken as intending to perform some act that would be more appropriate for a sex, kink, or other "special" party.
Person A: I'm going down to get a drink of lemonade, do you want some?
Person B: Dude, it's not that kind of party!