"Nucular" is an intentional misspelling of the word "nuclear" which is usually intended to express contempt or amusement towards the commonplace noo`kyu-ler pronounciation of that word.
This pronunciation is disapproved of by some who consider it a mispronunciation, although at least one leading American dictionary considers it acceptable. One explanation for this change in pronunciation is that a familiar phonetic pattern is influencing a less familiar one. Many common adjectives in English do end in -cular, such as circular, molecular, and particular. This kind of analogy often gives rise to metathesis.
The pronounciation is in widespread use in the United States, and is occasionally heard from British and Canadian speakers. It is used in both common speech, and by educated speakers including lawyers, congressmen, and presidents.
A possible explanation for the pronunciation nucular is the prevalance of other -cule nouns and -cular adjectives in English and science (molecular, etc.) and the rarity of -cleus or -clear words. The Latin word nuculeus or nucleus means "kernel" and is derived from nucula, "little nut" (from nux + ula). "Nucular" would apparently be the adjective form of the "little nut" root; "nuculear" (so far unused) would be the adjective of the alternative spelling of "kernel".
