-1: English is not the richest lexicon ever to have existed; quite the opposite: it is notorious for its relative lack of range and precision when compared to many other languages. This is the point being touched on when, for example, adult learners of English complain that they can't adequately express their feelings because English speakers use the same word ("love") to describe how they feel about an old friend, how they feel about their spouse, and how they feel about a cheeseburger.

This implies only that it is impervious to a specific subset of defenses, those which deflect the attack. The item could still be defendable by ablation or other attack-absorbing defenses.

@AndrewLeach That's true of "undefendable", but "indefensible" is used with somewhat more versatility, perhaps because its deconstruction more vaguely implies non-association with a defense, whereas the deconstruction for "undefendable" is rather less ambiguous.

A peak would most likely be "of" the mountain, though it may also be "on" the mountain, but certainly not "in" the mountain.