Where does “legkellőbb” come from?

legkellőbb (Hungarian) comes from Hungarian kellőbb, from Hungarian kellő, from Hungarian kell, from Proto-Finno-Ugric kelke- — to be necessary, need to, must, be obligatory.

Ancestry of “legkellőbb”, step by step

Step	Language	Word	Meaning
1	Hungarian	kellőbb	—
2	Hungarian	kellő	present participle of kell; required, adequate,...
3	Hungarian	kell	must, need to, have to; to be needed
4	Proto-Finno-Ugric	kelke-	to be necessary, need to, must, be obligatory

Every word from Proto-Finno-Ugric kelke- →Every word from Hungarian kell →Every word from Hungarian kellő →