Where does “þat” come from?

þat (Old English) comes from Proto-Germanic þat, from Proto-Indo-European tód, from Proto-Indo-European to-, from Proto-Indo-European só — this; that.

Ancestry of “þat”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	þat	nominative/accusative singular neuter of *sa
2	Proto-Indo-European	tód	demonstrative pronoun; that; it
3	Proto-Indo-European	to-	it; that; the, that
4	Proto-Indo-European	só	this; that

Words derived from “þat”

that
t'
that

Every word from Proto-Indo-European só →Every word from Proto-Indo-European to- →Every word from Proto-Indo-European tód →