Is it common to use “thineselves” to say “yourselves”?

And this person didn't say "THOU" best prepare thyselves? To be pretentious one must endeavor to do so properly and with appropriate decorum.

Only if you're in a Shakespearean play Or trying to pretend to be old fashioned and fancy sounding. In other words: no. It's archaic. Along with thee, thou, thy, thine, etc.

No. They used that in the 1600s, not now. 

No, it's archaic. Old. Not common at all. https://en.wikipedia.org/wiki/Thou

It's a term inspired by Shakespearean/Early Modern English, used as a joke.

It’s not even a correct archaic word.

"Thinesleves" is incorrect for two reasons, first because it should start with "thy" and not "thine", and second because thy is exclusively singular, and cannot be used to refer to more than one individual. (That is, the correct form can only be "thyself".) Third, that sentence in particular is wrong because you would never use "you" and "thy" to refer to the same person at the same time. Finally, "thy" in all its forms is archaic, started falling out of fashion about four hundred years ago, and has basically been extinct for over two centuries. In other words, "thineselves" is a sign that someone is *trying* to sound old-timey or archaic but has no actual understanding of how that earlier stage of the language actually worked.

No, it's an archaic word. If I saw someone use "thineself" I'd assume they were using it in a joking manner.

Nope. It’s archaic and occasionally used for flavor. Most of us know it mainly from old sayings like “to thine own self be true”

In exactly one situation. There's an alchoholic recovery organization called "alchoholics anonymous" where people both gather together to talk out the problems they're having in recovery and they'll also give you someone called "your sponsor" who you can call if you're having trouble. Their motto is "to thine own self be true" basically as a mantra to remind yourself "you want to not be addicted to alchohol, stick with it" It was also founded in 1935 and the motto comes from 1935 so the construction is really archaic by now. "Thine, thee, thou" are all not used in common english.

No, it's not common and I don't even think it's a word. I think the person who wrote this meant to use the archaic word "thyselves". Nobody uses that anymore, though.

Thy/thou and thisen are relatively common in the Yorkshire dialect. Although as far I am aware no other English dialect still uses it. “You best prepare thisen” would make sense

No. "Thou" and all of its derivatives are considered archaic terms.

It’s a pseudo-archaic word, and using it in that way is ungrammatical. The actual archaic words it’s based on are *thine* and *thyself.* The writer is mixing old and new structures, possibly for humor.

It's always for a joke or dramatic effect

"Thineself" does not exist. "Thyself" does exist, but is considered to be an archaic word in most dialects except a certain few in England, as far as I'm aware.

The closest I ever hear to that as an American English speaker is when someone quotes Shakespeare. "To thine own self be true". Even then it's pretty rare.

"Thine" is the Early Modern English version of "your(s)". No one has used it for at least 300 years or more, unless they're purposefully trying to sound like a historical figure (eg, for comedic effect) https://en.m.wikipedia.org/wiki/Thou

What! Dost thou consider not the normalcy of thineselves? Begone with you heathenous wraith! Befoul my tongue and mind alike no more!