Yet again, you have no legitimate or substantive rebuttal to anything else I've said, this time the only thing you are able to do is pick out one single word and dispute whether or not YOU feel it is proper usage. Talk about missing the point and focusing on a completely irrelevant detail, a typical "smokescreen tactic" used by people who have nothing relevant or informative or accurate to say that is actually related to the original topic of discussion.

BTW, there is precedent in the use of the word "theirself", despite the fact that it is not in common usage today:
"In Standard English, the pattern of reflexive pronoun forms shows slightly irregular patterning; all forms but two are composed of the possessive form of the pronoun and -self or -selves, as in myself or ourselves. The exceptions are himself and themselves, which are formed by attaching the suffix -self/-selves to the object forms of he and they rather than their possessive forms. Speakers who use hisself and theirselves are smoothing out the pattern's inconsistencies by applying the same rule to all forms in the set. · A further regularization is the use of -self regardless of number, yielding the forms ourself and theirself. Using a singular form in a plural context may seem imprecise, but the plural meaning of ourself and theirself is made clear by the presence of the plural forms our- and their-."
http://www.thefreedictionary.com »

OK, enough of this, you and I will apparently always be at odds, and although I'm sure there are some readers who are mildly entertained, most readers are probably good & sick of this exchange by now. You go ahead and consider yourself the winner, and I will consider myself the winner, and we can call it good. Or you can try taking another shot, and I'll ignore it.