Where does “txar” come from?

txar (Basque) comes from Basque -zar, from Basque zahar, from Proto-Basque *za(h)ar.

Ancestry of “txar”, step by step

Step	Language	Word	Meaning
1	Basque	-zar	—
2	Basque	zahar	old, aged
3	Proto-Basque	*za(h)ar	—