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Richard-Business Eng
Professional Teacher
So much confusion from two little letters... the prefix in-

in-

The prefix in- means ‘not, no, opposite of, or without’

Often, when a word starts with in-, we can assume that it means the opposite of the same word without the in-.

It’s easy to see that:
- the opposite of correct is incorrect
- the opposite of visible is invisible

OK… No problem, right?

BUT… there are a few English words that defy rational interpretation.
Some words that are written with the prefix in- simply don’t make sense.

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Famous and Infamous

Infamous does not mean “not famous”… What?

Definitions:
Famous:           widely known, well-known, having a favourable reputation, esteemed
Infamous:        known widely and usually unfavourably; extremely bad reputation
                          if someone is famous for a negative reason, they are infamous

So – Infamous does not mean ‘not famous’.

Example sentences:
- The pop star was so famous that he could hardly go anywhere without being identified.
- Criminals are often infamous because of their well-known negative reputation.

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Habitable and Inhabitable

Definitions:
Habitable:        means able to be lived in; suitable for human dwelling
Inhabitable:     means the same as habitable.

Inhabitable is a synonym of habitable.
Inhabitable does not mean ‘unsuitable for living in’.

Example Sentences:
- Our new house is much more habitable/inhabitable than our old house.

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Valuable and Invaluable 

Invaluable does not mean “having no value”… What?

Definitions:
Valuable:       having great material or monetary worth – having merit
Invaluable:    having worth so great that it can’t be quantified – priceless… (NOT having no value).
                       having incalculable monetary, intellectual, or spiritual worth

So, if something or someone is invaluable that doesn’t mean that it/they have no value.
Invaluable means it is usually of the type of value that cannot be counted.

Example:
- The help she gave him before his spelling test was invaluable – he passed with full marks.

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Flammable versus Inflammable

Flammable substances and inflammable substances burn… What?

Definitions:
Flammable:      easily ignited
Inflammable:    easily ignited

These are both synonyms

The word ‘inflammable’ predates ‘flammable’.
1400s – Inflammable came into use
1800s – flammable came into use

The prefix ‘in-‘ meant that the word inflammable would mean “not capable of burning”.
But its original use meant that the substance or object could burn.
It is entirely possible that the word ‘flammable’ became popular due to people assuming that ‘inflammable’ meant ‘not flammable’.

So we now use “flammable” to make it absolutely clear something may be dangerous.

Example:
- Be careful when filling your car – petrol (BrE)/gas (AmE) is highly flammable.

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Apr 14, 2018 11:46 PM
Comments · 7

Good observation :-). Actually English "in-" mixes up two Indo-European prefixes with very different meanings:

(1) negative in-, related to the negation non, German un- and Greek a(n)-

(2) directional in-, related to the prepositions in, into

Inflammable is an example of prefix #2, to burst into flames. Just like innovate, invigorate, initial, invoke, etc. Inhabitable comes from the same source: you can live in there.

Infamous is an example of prefix #1; a good explanation would be that fama means a good reputation, just like its descendant "fame" - thus, infamous becomes "disreputable" and that's logical again ;-).

Working tirelessly to restore (our faith into) sense in the universe. :-P

April 15, 2018

Thank you Richard and Aurelio for the explanation about the origin of the two different prefixes.

As a native speaker of a Romance language only two words are really weird for me:

1) inhabitable which in Italian/French would rather mean « not suitable to live in it » 

2) irregardless which seems to have a double negation.

April 15, 2018

And that one also has the dual origin, e.g. irradiate (shine onto something) and irrigate vs. irrational and irreverent :-).

For il-words with sense #2, illuminate comes to mind, for im- with the meaning "into" we have e.g. immigrate, immolate, impact and also impeach (from the same root as impediment), probably many others :-)

Sadly, this does not fix the nonsensical nature of "irregardless" - that one remains illogical, no matter what you do :-). I suspect it is a cross between irrespective and regardless.

April 15, 2018
Oops.  I forgot about the prefix ir-.  See above in Richard's comment.
April 15, 2018

We also have the prefixes il- and im- in English.  They are the same thing as in-.  The only difference is because of the sound that comes after the prefix.  What Aurelio said above holds true here also.  Here are a few examples:

Illogical = it makes no sense

illegal = against the law

illegible = you can't read it

illicit = not permitted

illiterate = you can't read

immaculate = totally pure and clean ( The root parts of the word maculate would mean something like having spots, as far as etymology. There is a book called The Maculate Muse.)

immortal = it will never die

impartial = unbiased

April 15, 2018
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Richard-Business Eng
Language Skills
English, French
Learning Language