The word hippopotomonstrosesquipedalian ["Of or pertaining to extremely long words"] and the even more dubious hippopotomonstrosesquipedaliophobic ["(humorous) The fear of long words"] can be found online, but neither appears in any reputable dictionary; indeed, Wiktionary notes of the latter that "It is unlikely that this 15-syllable contrivance is ever used purely for its meaning".
The longest noun formed by "stacking -o adjectives* and found in a reputable dictionary is:
pneu·mo·no·ul·tra·mi·cro·scop·ic·sil·i·co·vol·ca·no·co·ni·o·sis | \ ˈn(y)ü-mə-(ˌ)nō-ˌəl-trə-ˌmī-krə-ˈskäp-ik-ˈsil-i-(ˌ)kō-väl-ˈkā-nō-ˌkō-nē-ˈō-səs \
plural pneumonoultramicroscopicsilicovolcanoconioses \ -ˌsēz \
Medical Definition of pneumonoultramicroscopicsilicovolcanoconiosis:
a pneumoconiosis caused by inhalation of very fine silicate or quartz dust
[Source: Merriam-Webster Dictionary]
No adjectival form is offered, but one recognised way to form an adjective from a word with the suffix -osis is to use the suffix -otic: e.g. osmosis (osmotic), neurosis (neurotic), thrombosis (thrombotic). This would produce the adjective pneumonoultramicroscopicsilicovolcanoconiotic.
Wikipedia's entry for the noun provides an etymology:
This word was invented in the annual meeting of the National Puzzlers' League (N.P.L.) by its president Everett M. Smith. The word featured in the headline for an article published by the New York Herald Tribune on February 23, 1935, titled "Puzzlers Open 103rd Session Here by Recognizing 45-Letter Word"
Pneumonoultramicroscopicsilicovolcanoconiosis succeeded electrophotomicrographically as the longest word in the English language recognized by the National Puzzlers' League at the opening session of the organization's 103rd semi-annual meeting held yesterday at the Hotel New Yorker. The puzzlers explained that the forty-five-letter word is the synonym of a special form of pneumoconiosis caused by ultra-microscopic particles of silica volcanic dust...
Subsequently, the word was used in a puzzle book, Bedside Manna, after which time, members of the N.P.L. campaigned to include the word in major dictionaries.
This 45-letter word, referred to as "P45", first appeared in the 1939 supplement to the Merriam-Webster New International Dictionary, Second Edition.