An ad hoc assumption or adhoc hypothesis is a way of explaining away your wrongness.
After you discover that you are mistaken, you make an assumption that, if true, allows you to conclude that you were not really mistaken
If I run an experiment in which I predict some psychic phenomena, but the results contradict my hypothesis, I can argue that my psychic phenomena still exists, but the problem is that there were skeptics who were observing the experiment, and they sent off bad psi vibes (or some such thing).
We believe that Joseph Smith is a mouthpiece for God.
Observation: He was wrong about men on the moon.
Ad hoc assumption: he was speaking as a man.
We believe Joseph Smith was a translator:
Observation: Book of Abraham
Ad hoc assumption: he didn't literally translate it, it was simply a tool of inspiration
We believe we will be blessed for paying tithing.
Observation: You are a bankrupt
Ad hoc assumption: God is testing you
We believe we can be healed by blessings
Observation: non healing
Ad hoc assumption: the recipient lacks faith
We believe that the church takes tithing & offerings for purposes of charity
Observation: There is little evidence that this is true
Ad hoc assumption: the church doesn't want to brag about it's charitableness
We believe that black people are cursed, and were less valiant in the pre-existence
Observation: 1978 revelation
Ad hoc assumption: that was simply a policy, never a doctrine
by Fetal Deity
Apologists are pseudo-scientists. That is, they do their "science" in reverse. They start with a conclusion. In the case of Mopologists, that means, for instance, that they have concluded ("spiritually") that the BoM is the word of God. Then, they look for evidence that supports their conclusion; any evidence that contradicts their previously arrived-at conclusion is either ignored, distorted or completely redefined.
Bottom line: it is impossible to engage in a rational, objective argument with an apologist in the area he or she is defending ... IMPOSSIBLE ... as in: DON'T WASTE YOUR TIME!!!