good-humoured/tempered, in a good mood

The origin of this adjective goes back to the days when the type of headgear people were wearing (and how they were wearing it) could be indicative of their mood. Literally it meant: wearing the hat well. Note that the Dutch noun ‘muts’ no longer is a general word for headgear; it typically refers to the kind of hat that you can pull over your ears and is made of wool, or a wool like fabric, see DWOTD Muts.

‘Goedgemutst’ is slightly colloquial; a common more formal synonym is ‘goedgehumeurd’.

“Zo, wat ben jij goedgemutst vandaag!” – “Klopt, vandaag is de eerste dag van mijn vakantie!” 
(“You are in quite a good mood today!” – “That’s right, today is the first day of my holiday!” Note that ‘vakantie’ is typically used for a longer period of time-off.)

“Zij is echt prettig gezelschap, altijd goedgemutst en daar word je zelf ook heel vrolijk van!” 
(“She is really pleasant company, always good-tempered and that really cheers you up as well!”)

“Ik geef toe, ik ben niet echt het goedgemutste type bij uitstek, maar om nu te doen alsof ik altijd depressief ben, dat vind ik ook weer zo wat!” 
(“I admit, I’m not really an outstanding example of the good-humoured kind of guy, but to act as if I’m always depressed, that’s a bit too much to my liking.”)

Wat is er aan de hand? Vanochtend was je nog zo goedgemutst!” – “Ach houd toch op met dat goedgemutst, het is pokkenweer en het is koud! Daar word ik chagrijnig van! Laat me met rust!” 
(“What’s the matter? This morning you were in such a good mood!” – “Ah stop the ‘in such a good mood’ will you, it’s lousy weather and it’s cold! That makes miserable! Leave me alone!”)

Related words:
Muts: hat, cap [noun] [de muts, de mutsen].
– Goedgehumeurd: good-humoured [adjective].
– Humeur: temper, mood [noun] [het humeur, <no plural>].
– Stemming: mood [noun] [de stemming, de stemmingen].
Bui: mood [noun] [de bui, de buien].
– Vrolijk: cheerful, merry [adjective/adverb].
Blij: happy, glad [adjective].
– Opgewekt: cheerful, lighthearted [adjective].