What does insidious mean?
Word: Insidious
Part of Speech: Adjective
Meaning: Insidious means spreading slowly and quietly, hidden at first, while causing real harm.
How do you spell insidious?
Syllables: in - sid - i - ous
Tricky part: The ending is where pupils go wrong. They write eous, or they drop a vowel altogether, because the sounds run together quickly.
Watch out for: insideous, insidous
Memory tip: Let one small i slip in quietly before ous, just as the word itself describes things slipping in unseen.
Insidious in a sentence
Example 1: Noise in a classroom has an insidious effect on concentration.
Example 2: Termites do insidious damage because they stay out of sight.
Example 3: Salt air is insidious: it eats into the metal railings along Marine Drive.
Fun Fact
The Latin behind it grew from a verb for sitting, so the picture is of something that settles in quietly and stays out of view.
Practise the CREST Way
Word Cookie
Find the extra letter in the word:
insiddious
a) n
b) u
c) s
d) d
Show answer
d) d. One d is all it takes; the word is said with a single soft d.
Mingle-Mangle
Unjumble the letters in the sentence and choose the word:
The ssundiiio spread of weeds along the canal was not spotted for years.
a) industrious
b) insidious
c) ingenious
d) invidious
Show answer
b) insidious. Two s's and three i's: no other option can match that.
Word Connection
Choose the antonym of the word insidious as used in the sentence:
Damp is an insidious problem in old buildings.
a) sly
b) cunning
c) obvious
d) subtle
Show answer
c) obvious. A problem everyone can spot at once is the opposite kind.
Spell-o-Spell
Only one of these words is misspelt. Choose its meaning.
1. gradual 2. insidious 3. corosive 4. relentless
a) doing harm before anyone notices
b) tending to eat slowly into metal
c) refusing to stop or grow weaker
d) happening little by little
Show answer
b) tending to eat slowly into metal. Corrosive needs a double r.
Multiple Choice Questions
1. Why do engineers call rust an insidious enemy of bridges?
a) it turns the metal bright red
b) it works out of sight for years
c) it washes off in heavy rain
d) it makes a loud noise at night
Show answer
b) it works out of sight for years
2. Which habit could fairly be called insidious?
a) losing sleep a little more each week
b) arriving an hour late just once
c) forgetting a pen on exam day
d) dropping a plate on a stone floor
Show answer
a) losing sleep a little more each week
