Where does “nejít” come from?

nejít (Czech) comes from Czech jít, from Old Czech jíti, from Proto-Slavic jьti, from Proto-Balto-Slavic eitei, from Proto-Indo-European h₁éyti, from Proto-Indo-European h₁ey- — to go.

Ancestry of “nejít”, step by step

nejít traces back through two separate lines of ancestry.

via Czech jít

Step	Language	Word	Meaning
1	Czech	jít	to walk, go; to work, do, suffice; can, to be...
2	Old Czech	jíti	to go, to move; to walk
3	Proto-Slavic	jьti	to go
4	Proto-Balto-Slavic	eitei	to go
5	Proto-Indo-European	h₁éyti	to go, to be going
6	Proto-Indo-European	h₁ey-	to go

via Czech ne

Step	Language	Word	Meaning
1	Czech	ne	no!; not
2	Old Czech	ne	Negation particle; no, not
3	Proto-Slavic	ne	not
4	Proto-Balto-Slavic	nē	not
5	Proto-Indo-European	né	not; no

Every word from Proto-Indo-European h₁ey- →Every word from Proto-Indo-European h₁éyti →Every word from Proto-Balto-Slavic eitei →