'preocupar' originates from Late Latin, specifically the word 'praeoccupare', where 'prae-' meant 'before' and 'occupare' meant 'to seize or occupy'.

'praeoccupare' passed into Late Latin/Medieval Latin and then into Old Spanish as 'preocupar', eventually becoming the modern Spanish verb 'preocupar' (and reflexive 'preocuparse').

Initially it meant 'to occupy beforehand' or 'to take possession in advance', but over time it evolved into its current meaning of 'to cause anxiety / to occupy the mind (with concern)'.