Where does “fløthæ” come from?

fløthæ (Old Danish) comes from Old Norse flautir, from Proto-Germanic *flautijǭ.

Ancestry of “fløthæ”, step by step

Step	Language	Word	Meaning
1	Old Norse	flautir	sour whipped milk; something that floats on top
2	Proto-Germanic	*flautijǭ	—