Title says it all.
Example: un-freaking-believable.
Man, two minutes!
Thanks. Coincidental that I used the same example.
BTW dystmesis is the same, but with the extra word inserted in an unlikely place (so to speak): “unbe-freakin-lievable”
I can’t be the only one who thinks unbe-freakin’-lievable is the right place for it to go. In fact, Wikipedia backs me up as to un-freakin’ believable being the less regular way. After all, expletive infixation does obey certain rules (see cite).
Right. You cannot insert a word into a metrical foot. “Unbelievable” breaks up as (un.be.)(lie.va.)(ble.)
Native speakers do that all the time, so yes, you can.
Okay, so what’s the term for things like “bass-ackwards”?
Come to think of it, are there any other things like “bass-ackwards”?
Spoonerisms, especially if unintentional.
Wow, exactly the same point with exactly the same example was given five years ago in this thread.
Oh, I’m not saying you should have found it. I just remembered it because steno is my brother, and as far as I know that was his only thread! 
Can anyone think of examples where the inserted word isn’t either a cuss word or a euphemism for a cuss word?
{Talking about infixing “bloody”}: “The word went through, for the first time in the history of a language of the Indo-European family, a grammatical and syntactical process that is known only to several American Indian languages and to on or two others. That is the process of infixing, in which a particle, or a part of a word, or a whole word, is placed in the middle of another word…”
Ashley Montague, The Anatomy Of Swearing
As far as I know, adjectival swearwords like “bloody” and “fucking” are the only ones that are English infixes.
In American english, “fucking” and variants of it are the only infixes.
I don’t know about that. I’ve heard people confused by where to break the word “another” say things like, “That’s a totally nother problem!”
Are they native speakers of English?
It is probably an error. “That’s a totally nother problem!” should be “That’s totally another problem!” or “That’s a totally different problem!” They seem to have collided.