Where does “mertain” come from?

mertain (Ingrian) comes from Ingrian -in, from Proto-Finno-Ugric *-in, from Proto-Uralic -ma.

Ancestry of “mertain”, step by step

StepLanguageWordMeaning
1Ingrian-inForms adjectives indicating the material of which something is made; -en
2Proto-Finno-Ugric*-in—
3Proto-Uralic-ma—
Every word from Proto-Uralic -ma →Every word from Ingrian -in →Every word from Proto-Finno-Ugric *-in →
mertain — Etymology, Origin & Word History | Etymology Explorer