The spelling of "i bahn" using IPA phonetic transcription would be /ˈiː bɑːn/. This transcription shows that the first sound is a long E sound, followed by the "bah" sound, which is spelled "b-a-h" but pronounced as "bɑː". The "n" at the end is pronounced as a standard English "n" sound. This spelling, although unusual in English, accurately represents the pronunciation of the phrase, which could be translated to mean "I am going" in German.
The term i bahn does not have a well-established etymology, as it seems to be a slang phrase or a specific term used in certain contexts. However, based on context, it appears to be a humorous, non-standard way of saying I ban or I'm banning.
If you were referring to the German term U-Bahn (short for Untergrundbahn), which means underground train in English, its etymology can be traced back to the German language. The word Bahn means rail or track, and U stands for Untergrund, meaning underground.