Inhospitable, involuntary, invisible, incoherent, intolerable … It seems intuitive and it’s often inferred that “in-” words mean the inherent opposite — or inverse — of the word that “in-” precedes. Or maybe not. “In-” does not invariably indicate “not”; decapitate the “in-“, and you’re left with an invalid, incomprehensible word (see all the bold “in” words in this sentence). Inherit, incense, incumbent, invite and intrude are just a few further examples of the innumerable in-words that are unviable (yes, that is a word, but inviable is not) without their first two letters. And in a very few instances, when you take away the “in-“, you have a synonym — or something that comes in-terestingly close in meaning to the original intact word. Go figure.
Famous and infamous: mean almost the same thing. F = celebrated; well-known. I = notoriously bad (and notorious means well-known, especially unfavorably).
Habitable and inhabitable: mean the same thing. Both mean that can be inhabited.
Valuable and invaluable: mean very similar things. V = of great value, price or worth; I = valuable beyond estimation, priceless.
Fix and infix: can mean the same thing. F = mend, repair, make firm or stable; I = fix (a thing in another).
Dent and indent: can mean the same thing. D = mark with a dent. One definition of I = make a dent in.
Flammable and inflammable: mean exactly the same thing. As the OED notes: “Flammable is used because inflammable can be mistaken for a negative (the true negative being non-flammable). Inflammable, the original word that was in standard English usage from the 16th century, derives from the Latin inflammare meaning to kindle or set alight. Flammable was deliberately introduced and its use encouraged in the 1920s by the National Fire Protection Association, which was understandably concerned that the word inflammable was commonly being misunderstood to mean non-flammable or fire-proof.
Below are a few further examples of words that look as though they might or should have different meanings:
Disassociate and dissociate: mean the same thing: Both mean disconnect or become disconnected; separate.
Iterate and reiterate: mean the same thing. I = repeat; state repeatedly. R = say or do again or repeatedly.
Cogitate and excogitate: mean almost the same thing. C = ponder; meditate. E = think out; contrive.
Appertain and pertain: mean the same thing. Both mean to relate or be appropriate (to).