Where does “tir” come from?

tir (Old Cornish) comes from Proto-Celtic tīros, from Proto-Indo-European ters- — dry.

Ancestry of “tir”, step by step

Step	Language	Word	Meaning
1	Proto-Celtic	tīros	dry land
2	Proto-Indo-European	ters-	dry

tir — Etymology, Origin & Word History | Etymology Explorer