The adjective unassailable means without flaws or loopholes. If you are going to get home late (again!), you'd better have an unassailable alibi for your parents, or else you should plan on not seeing the outside of your room for a while.
The adjective unassailable also means immune to attack or doubt, such as an unassailable military holding or an unassailable reputation. Unassailable can also mean untouchable or unable to be defeated. If your high school basketball team is leading 92 to 14 with just a couple of seconds to go, the announcer might say you have an unassailable lead. The corrupt politician had so many connections, he seemed unassailable; he always seemed to manage to deflect even a hint of scandal onto others in the party.