I just realized I've played this game a bunch of times and I have no idea. International Flying Ship?
Well HMS stands for His/Her Majesty's Ship, so IFS would be... hmmm... erm... I have no idea! I would think your guess with International Flight or Flying Ship is pretty spot on, as far as I know there's not any in game information about the ship itself. Who even built it anyway?
This thread made me realise that I, too, never thought about decoding the abbreviation before. According to my knowledge, it was never explained in the game. I found myself unable to find anything sensible outside of it as well. There doesn't seem to be any real life IFS abbreviation related with blimps. I honestly wonder if it's supposed to mean anything at all or is it just a random sequence of letters placed there to make it look fancy. International Flying Ship would make sense, though according to a Google search, this thread is the first and only time anyone ever mentioned such a term on the Internet. Perhaps it's a minor unrevealed Easter Egg, initials of someone from the staff or his girlfriend, for example. Hieronymus Maxim was the blimp's constructor.
Oh right, yeah I some times miss out talking to him unless I'm a thief. My characters just don't care enough about Stennar Rockcutter to bother!
If Maxim was feeling a boastful he could have called it the Incredible Flying Ship! However inflatable does seem a bit more appropriately scientific.
That doesn't leave out the possibility it could be named after someone they started going out with on Second Life, IT guys love that shit.