What is the meaning of every letter of the word rabbit?

There is no meaning to any of the letters. Letters don't have meanings.

The individual letters in a Word don't mean anything. By themselves, they simply indicate what sound you're supposed to make. For example, the letter "s" indicates the s sound (which sounds like "ssssss")

The letters in the English alphabet do not have meanings. Only the Words that are made up of them do.