Where does “unless” come from?

unless (English) comes from English upon, from Middle English upon, from Middle English up-, from Old English ūp-, from Old English ūp, from Proto-Germanic upp, from Proto-Germanic ub, from Proto-Indo-European upó.

unless (English): Except on a specified condition; if not; If not;...

Definitions

Except on a specified condition; if not; If not;...

Ancestry of “unless”, step by step

unless traces back through 3 separate lines of ancestry.

via English upon

Step	Language	Word	Meaning
1	English	upon	Physically above and in contact with; Physically...
2	Middle English	upon	upon
3	Middle English	up-	—
4	Old English	ūp-	up-
5	Old English	ūp	up, from above
6	Proto-Germanic	upp	up, upwards
7	Proto-Germanic	ub	under
8	Proto-Indo-European	upó	under, below
9	Proto-Indo-European	h₃ewp-	—

via English of

Step	Language	Word	Meaning
1	English	of	API; Expressing distance or motion; "Expressing...
2	Middle English	of	of; off; until
3	Old English	of	of, from, off, out of
4	Old English	æf	of
5	Proto-Germanic	*ab	—
6	Proto-Indo-European	apó	—

via English in

Step	Language	Word	Meaning
1	English	in	Used to indicate location, inclusion, or position...
2	Middle English	in	Any kind of accomodation; particularly; A home or...
3	Old English	in	in; Alternative form of inn
4	Proto-West Germanic	*in	—

Every word from Proto-Indo-European h₃ewp- →