Why are not infamous and inflammable the opposite of famous and flammable, like incomplete, inactivity, inappropriate and so on?
The New Oxford American Dictionary I had on my Mac Mini (which was the third edition, last time I checked) reported the following definitions for in-:
1. (added to adjectives) not: inanimate | intolerant.
2. (added to nouns) without; lacking: inadvertence | inappreciation.
prefix in; into; toward; within: induce | influx | inborn.
Looking at the origin of infamous, and inflammable, I read the following:
ORIGIN late Middle English: from medieval Latin infamosus, from Latin infamis (based on fama 'fame').
ORIGIN early 17th century: from French, or from Latin inflammare (see inflame).
In both the cases, the words are not built adding the prefix in- to existing words.
(Just to be contrarian.)
The word infamous is the opposite of famous! Just as the opposite of reputed is disreputed rather than obscure, and the opposite of hot is cold rather than not hot, the opposite of famous (having "good" fame) is infamous (having "bad" fame, having infamy, ill-famed).
The word "flammable" is newer than inflammable and does not exist, for instance, in Indian English. Historically, the only word was inflammable, dating to at least the 16th century. "Flammable" did not exist. Note that we still say inflammatory speeches not *flammatory speeches, inflammation of the skin not *flammation of the skin, etc. (However, flammation "exposure to fire" actually exists in the OED and is marked as obsolete, the only quote dating to 1646.)
The word "flammable" was invented around 1813, but it didn't catch on, and some time in the 19th century was pretty much dead: The 1913 Webster's dictionary marks the word "flammable" as obsolete. Unfortunately (IMHO), this word was revived after World War II. (See this letter. More precisely, the word flammability was revived, and then "flammable" followed.)
The OED entry for flammability is:
= inflammability n. Revived in modern use to avoid the possible ambiguity of inflammability, in which the prefix in- might be taken for a negative (in- prefix3).
In my opinion, the word flammable was unnecessary: there's not much confusion possible in seeing, on a gas/petrol tanker, the words "Warning: Highly inflammable". It seems to me there's more confusion about the meaning of inflammable when the word "flammable" exists, than when it doesn't. But where "flammable" is already common enough, it's safe—and recommended—to use it.