I'm surprised prefix in- can add opposite meaning: (not) inactive, indefensible vs (extra) invaluable, inflammable. Are there any rules to intelligently guess its meaning for newly learn word? The topic was difficult to web-search due to ubiquitous nature of in.

  • I would understand invaluable to mean not able to be valued - so useful that you can't put a price on it. Aug 23, 2021 at 7:20
  • The prefix "in" doesn't mean "extra". In "invaluable" it means "not" (as KB points out), while in "inflammable" it originally meant "into" (i.e., an inflammable object could burst into flames). (However, look up the etymology of "inflammable" for some interesting history.) May 9 at 2:51

1 Answer 1


Unlike "un-", the "in-" prefix is not very productive now. That means that while there are lots of words that use "in-", it isn't being attached to new words. Most of the "in-" words are from Latin or from Latin via French, and may have picked up special meanings over the years. So "invaluable" means "priceless" and not "worthless". This is part of the meaning of the word and can't be guessed.

A few, like "inflammable" have a sense that comes from "in" (ie inside) and not from a root "*n-" or "*ne-" meaning "not".

Generally, words with "in-" prefix just mean "not ..." except when they don't.

  • "indefensible" does not look as from Latin, as you wrote - just an old word, an exception? Aug 24, 2021 at 4:12
  • 1
    defense is from Old French defense, from Latin defensus,
    – James K
    Sep 6 at 5:40

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