'alterarse' originates from Latin, specifically the word 'alterare', where 'alter' meant 'other'.

'alterarse' changed from the Latin word 'alterare', passed through Old French/Medieval forms such as 'alterer' and the Romance developments, and became Spanish 'alterar' with the pronominal/reflexive form 'alterarse'.

Initially, it meant 'to make different or to change', but over time it evolved to include senses such as 'to become upset' and 'to spoil/go bad' in reflexive use.