'delouse' is formed from the prefix 'de-' (from Latin 'de-' meaning 'remove, away') + the noun 'louse' (Old English 'lūs') meaning 'a small parasitic insect (louse)'.

'louse' comes from Old English 'lūs', from Proto-Germanic '*lūsą'; the verb form developed in Middle English (e.g. Middle English 'delousen') and eventually led to the modern English verb 'delouse' by affixing 'de-' to indicate removal.

Initially it literally meant 'to remove lice', and this primary meaning has been retained into modern usage (to rid of lice or similar parasites).