'insect-ridden' is a compound of 'insect' and 'ridden'. 'Insect' originates from Latin, specifically the word 'insectum', where the prefix 'in-' meant 'into' and the root 'secare' (in classical formation) meant 'to cut' (referring to segmented bodies). 'Ridden' is the past participle of 'ride', from Old English 'rīdan'.

'insect' passed from Latin 'insectum' into Old/Middle French as 'insecte' and then into Middle English as 'insect'. 'Rīdan' in Old English produced the past participle 'ridden' in Middle English; the adjectival use seen in compounds like 'disease-ridden' or 'ridden' (meaning 'afflicted by') developed in Modern English, and compounds such as 'insect-ridden' follow this pattern.

Initially 'insect' in Latin referred etymologically to the idea of being 'cut into' (reflecting segmented bodies), and 'ridden' originally meant 'mounted on' or 'driven'; over time 'ridden' acquired a figurative sense 'afflicted by' or 'full of', so the compound came to mean 'infested with insects'.