'washaway' is a compound formed from the verb 'wash' and the adverb 'away'. 'wash' originates from Old English 'wæscan' meaning 'to wash' and ultimately from Proto-Germanic roots meaning 'to wash, bathe'; 'away' originates from Old English 'awæg' / 'on weg' meaning 'away' or 'from a place'.

'wash' came into Middle English from Old English 'wæscan' and developed into the modern verb 'wash'; 'away' developed from Old English forms such as 'awæg' and 'onweg'. The compound sense 'wash away' (phrasal verb) has been in use for centuries; the one-word form 'washaway' as a noun (meaning a washed-out area) is a later formation used in engineering and informal contexts (19th–20th century).

Originally, 'wash' meant simply 'to cleanse by water'; combined with 'away' it took on the sense 'to carry off by flowing water'. Over time, as a noun 'washaway' came to denote specifically a place or thing that has been washed away (a washout or erosion), expanding from the basic action to the resulting damage or gap.