This is a tough one, as there are many extremely long words which just aren’t words and have been created just to be ridiculously long. The longest word which actually has a meaning is: antidisestablishmentarianism. (28 letters) Which means “opposition to the disestablishment of the Church of England”.
Unofficially, the longest word is: pneumonoultramicroscopicsilicovolcanoconiosis. (45 letters) Which means a lung disease caused by silica dust.
However, the name of a protein weighs up with a staggering 1913 characters! Of course, this “word” would never actually be used, but here it is anyway!
methionylglutaminylarginyltyrosylglutamylserylleucyl phenylalanylalanylglutaminylleucyllysylglutamylarginyl lysylglutamylglycylalanylphenylalanylvalylprolylphenyl alanylvalylthreonylleucylglycylaspartylprolylglycylisol eucylglutamylglutaminylserylleucyllysylisoleucylaspartyl threonylleucylisoleucylglutamylalanylglycylalanylaspartyl alanylleucylglutamylleucylglycylisoleucylprolylphenyl alanylserylaspartylprolylleucylalanylaspartylglycylprolyl threonylisoleucylglutaminylasparaginylalanylthreonylleucyl arginylalanylphenylalanylalanylalanylglycylvalylthreonyl prolylalanylglutaminylcysteinylphenylalanylglutamyl methionylleucylalanylleucylisoleucylarginylglutaminyllysyl histidylprolylthreonylisoleucylprolylisoleucylglycylleucyl leucylmethionyltyrosylalanylasparaginylleucylvalylphenyl alanylasparaginyllysylglycylisoleucylaspartylglutamylphenyl alanyltyrosylalanylglutaminylcysteinylglutamyllysylvalyl glycylvalylaspartylserylvalylleucylvalylalanylaspartylvalyl prolylvalylglutaminylglutamylserylalanylprolylphenylalanyl arginylglutaminylalanylalanylleucylarginylhistidylasparaginyl valylalanylprolylisoleucylphenylalanylisoleucylcysteinyl prolylprolylaspartylalanylaspartylaspartylaspartylleucyl leucylarginylglutaminylisoleucylalanylseryltyrosylglycyl arginylglycyltyrosylthreonyltyrosylleucylleucylserylarginyl alanylglycylvalylthreonylglycylalanylglutamylasparaginyl arginylalanylalanylleucylprolylleucylasparaginylhistidyl leucylvalylalanyllysylleucyllysylglutamyltyrosylasparaginyl alanylalanylprolylprolylleucylglutaminylglycylphenylalanyl glycylisoleucylserylalanylprolylaspartylglutaminylvalyllysyl alanylalanylisoleucylaspartylalanylglycylalanylalanylglycyl alanylisoleucylserylglycylserylalanylisoleucylvalyllysylisol eucylisoleucylglutamylglutaminylhistidylasparaginylisoleucyl glutamylprolylglutamyllysylmethionylleucylalanylalanylleucyl lysylvalylphenylalanylvalylglutaminylprolylmethionyllysyl alanylalanylthreonylarginylserine
Good luck remembering how to spell it.
