Something that's indefensible is inexcusable — there's no way to justify it. It's indefensible to take candy from a baby, or to give candy TO a baby, for that matter.
Indefensible behavior is just plain terrible, and it can't be explained away. It's indefensible, for example, or for one country to drop a bomb on innocent civilians in another country. Indefensible, "not able to be defended," comes from in-, "not," plus the Latin defendere and its sense of "allege in defense."