Incànus was the name he ( Maybe ) was given, very early after his arrival to Middle-Earth, in
Gondor. Well Gandalf did not say it was given to him in Gondor, but that it was what he was called in the South.In the early years of the T.A. the ancient Gondor stretched much further down South, than in the period where the tale of the great ring-war unfurled. Men from
Númenorë had explored the coasts of Middle-Earth far Southward, and settlements beyond
Umbar had been established. But they were the first to be absorbed in the years to come and
Harad; far or near, just means South. The name Incànus is / was a devised
Quenya name, it is not a directly and original Quenya, or
Sindarin, word but a constructed one, made of the Quenya elements in(id) = Mind and Kan/Cànu = ruler / governor / Chieftain. But, for whatever it is, it later became obsolete and was only remembered by the learned in Gondor. As a curiosum it can be mentioned that in the Thains Book, there was a note, saying that it is a form adapted to Quenya of a word in the tongue of the
Haradrim, simply meaning North-spy.