Where does “þannā” come from?

þannā (Proto-West Germanic) comes from Proto-Germanic þan, from Proto-Germanic þam, from Proto-Indo-European tóm, from Proto-Indo-European só — this; that.

Ancestry of “þannā”, step by step

Step	Language	Word	Meaning
1	Proto-Germanic	þan	then, at that time
2	Proto-Germanic	þam	—
3	Proto-Indo-European	tóm	—
4	Proto-Indo-European	só	this; that

Words derived from “þannā”

than
Than
þonne
þanne
then(ne)
thenne
than(ne)
than
thanne
than
wherethan
more-than
nuh
dan

Every word from Proto-Indo-European só →Every word from Proto-Indo-European tóm →Every word from Proto-Germanic þam →