elrevigi

A genuine build, and one of the best in the language. Elrevigi comes apart with nothing left over:

el (out of) + rev- (dream) + -ig- (cause it) + -i (verb)

To disillusion somebody is to bring them out of their dream. English needs a Latin negative prefix and a noun it borrowed whole; Esperanto uses a preposition, a noun any beginner already has from revo, and the commonest suffix in the language. Read the parts and the meaning is simply there, which is the whole argument for the way Esperanto builds words.

The -ig- is doing the work, so get it the right way round. -ig- means cause it and -iĝ- means it happens by itself, and here the pair matters more than usual because both members are in common use. Mi elrevigis lin is I disillusioned him; li elreviĝis is he lost his illusions. The noun elreviĝo is disillusionment - the experience, not the act - and it is far commoner in print than the transitive verb. The two suffixes and the transitivity rule behind them are set out in igi and iĝi.

Not the same as disappointing somebody. Disappointment is about an expectation that went unmet, and Esperanto reaches for seniluziigi or plain ĉagreni for it. Elrevigi is narrower and colder: it strips the illusion itself, and the person is usually better informed afterwards even if they are unhappier. A synonym is seniluziigi, built the same way out of sen and iluzio - two routes to one idea, which is normal in a language where anybody can build a word.