untenable

1 Something that someone tells you and they can't prove it.

2 Unable to substantiate with solid evidence.

3 unprovable

Example 1

Teacher: Give me your homework.

Student: Here's my homework; (pretends to hand it over) I wrote it on magic paper that only wizards can see and touch.

Teacher: That excuse is untenable. You still get a zero and detention.

Example 2

Person A: I'm telling you; Jesus really exists in the communion wafers of the Eucharist. Transubstantiation is irrefutable because sacred scripture and church doctrine supports it.

Person B: No, That's not irrefutable, that claim is untenable! The church has no palpable proof themselves, only St. Thomas Aquinas's word games. All that faith in your head has mushed the brain out of your ears.

Person A: Heathen! You know in your heart that God is real and that he loves you!

Person B: That's also untenable. But I'm not saying that I can prove you to be wrong.

untenable

Someone that is very pretty but just falls "this much" short of your dreams.

Granted, she was pretty and all, but she was unTENable.