Definition:   It describes something that is indefensible or unable to be defended. An example would be that a person’s thesis or argument is baseless and has no ground.

It can also mean a place that is not fit for dwelling, like an old condemned building.

Etymology: Untenable is from the Old French word tenir, which means “to hold or have possession of” and previously from the Latin tenēre “to hold, occupy, possess.” We use untenable when an idea or position is so unprovable that holding onto it is inexcusable.

Untenable entered English in the 17th century, and it meant “unable to be defended against attack.” That sense is still viable: you can use untenable to describe any position, situation, or theory that simply cannot be defended.

In a Sentence

With all of the natural disasters we have had this summer, life is becoming untenable for large populations in the nation.

The debate team ultimately lost the contest because their arguments were untenable.

Synonyms 

Insupportable, Unwarranted, Unsound or Indefensible

Antonyms 

Tenable, Defensible, Valid or Reasonable