Definitions:

1. incapable of being logically explained as a concept or idea adjective
   • The theory was indefensible, and was quickly dismissed by the scientific community.
2. not able to be protected against a literal attack adjective
   • The lack of a natural barrier made the city almost indefensible.
3. so horrible and distasteful to people that it's incapable of being justified or explained adjective
   • The way you treated them was absolutely indefensible.

Synonyms:

• insupportable
• unjustifiable
• untenable
• unwarrantable
• unwarranted