The word "theirself" is not a grammatically correct spelling in Standard English. Instead, the correct pronoun is "themselves". The IPA phonetic transcription for "themselves" is /ðəmˈsɛlvz/. This word is used to refer to a group of people or things previously mentioned. While "theirself" may be used in some dialects, it should not be used in formal writing or speech. It is important to use proper grammar and spelling in order to communicate clearly and effectively.
The word "theirself" is possibly a combination of the pronoun "their" and the reflexive pronoun "self". It emerged in the Scottish dialect in the early 16th century as a contraction of "their own self" or "their own selves". Over time, it gained limited usage in some dialects of English, particularly in Scotland and Ireland. However, "theirself" is not widely accepted as a standard word in modern English grammar. Instead, the reflexive pronoun "themselves" is commonly used to refer to a plural antecedent.