Dutch word regenbui comes from Dutch regen (Rain.), Dutch bui

Detailed word origin of regenbui

Dictionary entry	Language	Definition
regen	Dutch (nld)	Rain.
bui	Dutch (nld)	(dated) gust, current of wind. Mood. Shower (rain), or generally a bout (of bad weather such as snow).
regenbui	Dutch (nld)	Rain shower, incidence of rain.