c. 1400, "a box," especially, in Church use, the vessel in which the host or consecrated bread is kept, from Latin pyxis, from Greek pyxis "box-wood; a box" (originally one made of box-wood), from pyxos "box-wood; box-tree," a word of uncertain origin, perhaps a loan-word from Italy, where the tree is native.
In nautical use from 1680s as "the metal box in which the compass is suspended." Hence also Pyxis as the name of a Southern constellation proposed 1760s by Lacaille (along with Puppis, Carina, and Vela) to be made from parts of the unwieldy ancient Argo, though Malus "mast" also was long used for this part.