Moodypants
When someone or something is in a less than happy mood, this person may be described as moodypants, or alternatively, moodyplum.
Jo: If you jump over the fence to get that ball I'm going home.
Me: Don't be moodypants.
Jo: Fuck off.
Me: Moodyplum.
Me: Don't be moodypants.
Jo: Fuck off.
Me: Moodyplum.