Where does “unu” come from?

unu (Esperanto) comes from Esperanto nur, from German nur, from Middle High German nuor, from Old High German niwāri.

unu (Esperanto): one

Definitions

Step	Language	Word	Meaning
1	Esperanto	nur	only, just
2	German	nur	only, just, merely, simply; ever; at all;...
3	Middle High German	nuor	—
4	Old High German	niwāri	—

unu — Etymology, Origin & Word History | Etymology Explorer