Insidious

How to spell insidious

9 letters · 4 syllables

How to pronounce insidious

Say the part in capitals loudest.

What does insidious mean?

1. adjectiveProceeding in a gradual, subtle way with harmful effects that are not noticed until serious damage is done.

Insidious in a sentence

• The damp caused insidious damage to the floorboards long before anyone smelled the rot.

• High blood pressure is an insidious condition because patients often feel perfectly well.

Where does insidious come from?

Language of origin: Latin

The word comes from the Latin insidiosus, meaning 'cunning' or 'deceitful', which was built on insidiae, a noun for an 'ambush' or 'trap laid in waiting'. That noun in turn traces back to insidere, literally 'to sit in' or 'to sit upon', from in- 'in, on' and sedere 'to sit'; the image is of soldiers crouching out of sight, waiting to spring upon an unsuspecting traveller. English took the word in during the sixteenth century, by way of French, keeping the sense of harm that creeps up quietly rather than striking openly. The same sedere root that means 'to sit' still hides inside everyday relatives such as 'reside', 'sediment', and 'president'. Knowing that the middle of the word is the 'sid' of sitting in ambush is a useful way to lock in that tricky single 's' followed by 'id' before the '-ious'.

How the word is built

• in- (in, on)
• sid (from Latin sedere, to sit)
• -ious (adjective suffix)