Correct spelling for the English word "theirselves" is [θe͡əsˈɛlvz], [θeəsˈɛlvz], [θ_eə_s_ˈɛ_l_v_z] (IPA phonetic alphabet).
Theirselves is not a standard word in English. It is considered non-standard and incorrect grammar. The correct possessive pronoun in English is themselves. Theirselves likely originated as a colloquial variation or a combination of themselves and theirs.