Ximum is an early Modern Latin name for the Japanese island of Kyushu. The name is derived from the contemporary Western name for the island, Ximo, which is in turn derived from the Japanese word "下" (shimo, lower) — Kyushu being the southernmost of the main Japanese islands.

In 1698, Johann Jacob Hofmann wrote in his Lexicon Universale:

Dicitur Ximum, i.e., Regia inferior & Saycock, i.e. novem regna, quot continet: Nempe Figen, Bungo, Chicuien, Fingo, Fiunga, Bugen, Satcuma, Volumi & Uto.

(It is called Ximo, i.e., lower Court and Saycock, i.e. nine kingdoms, for as many as it contains: namely Hizen, Bungo, Chikuzen, Higo, Hyuga, Buzen, Satsuma, Chikugo, and Osumi.)

Reference: http://www.uni-mannheim.de/mateo/camenaref/hofmann/hof4/s0865b.html
I am guessing here on the identity of Volumi and Uto.

The explanation of Ximum is correct, though the explanation of Saycock is off: he is describing the origin of the name 九州 (Kyushu, nine provinces) itself, which doesn't appear in the entry. Saycock is probably 西国 (saikoku, western country) — Kyushu being the westernmost of the main Japanese islands.

(Notice the spelling of the names in the Latin, which predate Hepburn Romanization: f is common for h in transliterations of the time—it probably being still the contemporary pronunciation—and ng for g is a pronunciation still current in Japanese. "X" standing for "sh" is a feature of Portuguese and the Spanish of the time.)