'epistomatous' originates from New Latin/Greek, specifically from the Greek prefix 'epi-' (ἐπί) meaning 'upon' and the Greek noun 'stoma' (στόμα) meaning 'mouth' (used in biology for 'stoma'/'stomata'), combined with the adjectival suffix '-ous'.

'epistomatous' was formed in New Latin/modern scientific Latin from the Greek elements 'epi-' + 'stoma' and appeared in botanical and anatomical English usage as 'epistomatous' to describe the position of stomata on plant surfaces.

Initially formed from elements meaning 'upon' + 'mouth', it came to be used specifically in botanical contexts to mean 'having stomata on the upper surface' (i.e., 'stomata upon the leaf surface').