'marathon-watch' is a modern English compound formed from 'marathon' + 'watch'. 'Marathon' originates from Greek 'Marathṓn' (the place name 'Marathon'), while 'watch' comes from Old English 'wæcce'/'wæccan' meaning 'wakefulness' or 'to be awake'.

'marathon' entered English via Latin and French in the 19th century referring to the long-distance race named after the Battle of Marathon; 'watch' evolved from Old English 'wæcce' (vigil) and later came to mean a timekeeping device (modern 'watch' as a portable timepiece developed in the 16th–19th centuries). The compound 'marathon-watch' is a recent formation combining those two senses: either a watch for marathon running or the notion of a long-duration viewing.

Individually, 'marathon' originally referred to the place and then to the long-distance race; 'watch' originally meant 'vigil' and later 'timepiece'. Combined recently, the compound took on two related senses: a timepiece for marathon/running use, and (by extension of 'marathon' meaning 'long-duration') an event or action of long, continuous watching.