That makes it Shell Friendly Interactive Shell which is redundant. Then there would be Shell Korn Shell, and Shell C shell. The 'sh' for Shell goes at the end

That makes it Shell Friendly Interactive Shell which is redundant. Then there would be Shell Korn Shell, and Shell C shell. The 'sh' for Shell goes at the end as like a grammatical marker. It's postfix. Sh-FI-sh is awkward because it's prefix of the Shell -> 'sh' and also includes the 'sh' for Shell at the end making it a Shell X Shell sort of thing.