During (roughly) the first half of the 20th century, the most powerful naval vessels were all-big-gun battleships. The very first of these was HMS Deadnought and all-big-gun battleships were often called Deadnoughts, after the first of their kind. For many peoples, the first TT Capsule (the "correct" name for that kind of vessel) they encountered was the Doctor's, so it would be natural for them to apply the name TARDIS to all TT Capsules. TARDIS is also much less of a mouthful. Even the Time Lords would be likely to prefer the shorter name, unless they were speaking formally or technically -- or just being pedantic. As soon as the name started to catch on, it would be carried backwards in time, as well as forwards (these things are time machines, after all). Even in our society, it's often difficult to work out who first coined a name like that; in a time-travelling society like that of the Time Lords, it would be even harder. Susan's claim to have invented the name is supported, however, by the fact that TARDIS is derived from a phrase in the English language -- Time And Relative Dimension In Space -- not in Gallifreyan or any other language.