'preexisting' is formed from the prefix 'pre-' (from Latin 'prae' meaning 'before') + the verb 'exist' (from Latin 'existere' meaning 'to stand forth, emerge').

'pre-' + 'exist' traces to Latin 'prae' + 'existere'; through Medieval and Early Modern English the compound 'pre-exist' appeared, and the adjective 'preexisting' developed from that.

Initially the components meant 'before' + 'to exist'; over time they combined to express the idea 'to exist prior to a specified time or event,' which is the current meaning.