"Nucular" is an intentional misspelling of the word "nuclear" which is usually intended to express contempt or amusement towards the commonplace noo`kyu-ler pronunciation 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 Merriam-Webster dictionary has the following usage comment: [1]

"Though disapproved of by many, pronunciations ending in \\-ky&-l&r\\ have been found in widespread use among educated speakers including scientists, lawyers, professors, congressmen, U.S. cabinet members, and at least one U.S. president and one vice president. While most common in the U.S., these pronunciations have also been heard from British and Canadian speakers."

It should be noted that 'nucular' is also the preferred pronunciation of Homer Simpson, and his confident use of it in The Simpsons exemplifies a level of cultural information that is largely overheard at some distance, second-hand, rather than read. Correctness aside, the pronunciation is a revealing cultural shibboleth.

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".