Themself is a word that’s used as an alternative singular form of the reflexive pronoun or emphatic pronoun themselves. It’s used in combination with the singular ‘they’.
But there’s still some debate about whether this usage should be considered standard. Merriam-Webster lists it as ‘nonstandard’ but indicates that its use is increasing over time. APA Style regards it as an acceptable alternative to themselves but doesn’t require its use.
If you’re worried about correctness, our advice is to continue using themselves for both the singular and the plural sense for now.