The correct spelling of the word "was horizon" is actually "as horizon". The /w/ sound is not supposed to be there. The IPA phonetic transcription of the word is /æz həˈraɪzən/. The first sound is the short "a" sound as in "cat". The "z" is pronounced as in "buzz". The "h" is silent. The next sound is the short "i" sound as in "bit". The final syllable has the long "o" sound as in "go" and ends with the "n" sound.
There is a misunderstanding in your question. "Was horizon" is not a recognized term or phrase in English, so its etymology cannot be established. If you intended to ask about the etymology of the word "horizon", I can provide that information for you.