But the things she did during her run on the series cannot be denied, and that includes her nickname for the Doctor's time travel vehicle.
Susan claimed that she made up the name, but we've seen that it was being used on Gallifrey as well. Surely the Time Lords had a name for the vehicle beyond its classifications for the different types (Type 40, Mark IV, Mark III.)
I think "Tardis" is the actual Gallifreyan word for the coral-like growth that would become a time-hopping vehicle. What Susan did was to take the letters in that word and used them as an acronym, perhaps as a game for herself during that lonely time with just her grandfather as company. So for her, it became "T.A.R.D.I.S.", or as it's better known today, "TARDIS".
What "Tardis" actually meant in Old High Gallifreyan is unknown......