'diapsidan' originates from Neo-Latin 'Diapsida', specifically the taxonomic name 'Diapsida', where 'di-' meant 'two' and the element 'apsid/apsis' referred to an arch or opening.

'diapsidan' changed from the Neo-Latin taxonomic word 'Diapsida' combined with the English adjectival/noun suffix '-an' and eventually became the modern English word 'diapsidan'.

Initially it meant 'a member of the group Diapsida'; over time it has been used both as a noun for such animals and as an adjective meaning 'having two temporal skull openings'.