Hey there. The Latin prefix in- is fascinating to me because it represents at least two different meanings, and consequently has the 'power' to alter very powerfully the definition of any other word to which it is attached.
- in, into, on (prefix is also seen as im-, em-, or en-)
- not, occasionally, beyond belief
Let's call meaning (1) of in- the positive prefix, and meaning (2) of in- the negative prefix. This distinction should make our discussion flow a bit more smoothly.
Meaning (1) refers to a special grammatically intensive application of the prefix in-. In point of fact, this usage derives etymologically from the Latin preposition in.
Now then: An enormously confusing example of a word that employs the positive prefix of in- is the adjective inflammable. The OED defines inflammable as "capable of being inflamed or set on fire; susceptible of combustion; easily set on fire."
Etymologically, inflammable derives from the verb inflame. Inflame, in turn, derives from the following Latin formula:
in + flammare (verb, "to flame"), from flamma (noun, "flame")
What the heck is going on here? Does this mean that the adjectives inflammable and flammable are, in point of fact, synonyms?
Yes—the OED tells us so. Its definitional entry for flammable reads "= INFLAMMABLE. Revived in modern use: cf.prec."
In case you are wondering what "cf. prec." stand for, these cryptic OED abbreviations mean "compare with the previous." The "previous" in question here is the OED entry inflammable.
The following bit from the American Heritage Book of English Usage explains the distinction between inflammable and flammable quite succinctly:
Should you be careful with a solvent that’s inflammable? Absolutely. The trouble with flammable and inflammable is that they mean the same thing. The prefix in- is not the Latin negative prefix in-, which is related to the English un- and appears in such words as indecent and inglorious. The in- in inflammable is an intensive prefix that is derived from the Latin preposition in. This prefix also appears in the word enflame. But many people are ignorant of all this and conclude that, since flammable means "combustible," inflammable must mean "not flammable" or "incombustible." Therefore, for clarity’s sake, you should use only flammable to give warnings.
Let's now have a look at two more befuddling adjectives: valuable and invaluable. This first word means "of material or monetary value; having value for use or for exchange" (definition 1) or "having value or worth, of great use or service, to a person or for a purpose" (definition 2), according to the OED.
What's happening in this situation, folks, is that invaluable employs the negative Latin prefix in-, in the sense of "not."
Officially, the OED defines the adjective invaluable as "that cannot be valued; above and beyond valuation; of surpassing or transcendent worth or merit; priceless, inestimable."
In summary, invaluable = in- + valuable. That is to say, invaluable means "incapable of being valued."
There you have it. Enjoy your day.