Where does “undefensible” come from?

undefensible (English) comes from English defensible, from Old French defensable, from Latin defensibilem.

undefensible (English): Not defensible

Definitions

Step	Language	Word	Meaning
1	English	defensible	capable of being defended against armed attack;...
2	Old French	defensable	—
3	Latin	defensibilem	—

undefensible — Etymology, Origin & Word History | Etymology Explorer