From the Greek osto- (bone) and machia (battle). Lit., a battle with bones. Actually an ancient Greek game played with fourteen pieces of bone that sounds very similar to modern tangrams.
Ausonius wrote of it in a letter, saying:
"ossicula ea sunt: ad summam quattuordecim figuras geometricas habent. sunt enim aeqauliter triquetra: uel obliquis: isocele ipsi uel isopleura uocant, orthogonia quoque et scalena. harum uerticularum uariis coagmentis simulantur species mille formarum: helephantus belwa aut aper bestia, anser uolans et mirmillo in armis, subsidens uenator et latrans canis, quin et turris et cantharus et alia huiusmodi innumerabilium figurarum, quae alius alio scientius uariegant. sed peritorum concinnatio miraculum est, imperitorum iunctura ridiculum." (Decimus Magnus Ausonius, Opuscula, Peoper recension, XII, Technopaegnion, XVII, "Cento nuptialis", 42-54, p. 208 (Teubner; 1886).)
Or, roughly, "There they are little pieces of bone: having fourteen geometric figures in total. Some are equilateral triangles, or oblique, and also isosceles, scalene and right angles. By the joining of these together in various ways, they can be made to resemble all manner of things: a monstrous elephant or a bestial boar, a flying goose and an armed gladiator, a crouching hunter and a barking dog, even a tower and a tankard and innumerable other things of that kind, which vary according to the skill of this or that player. But while the compositions of the skilled are marvellous, the assemblings of the unskilled are laughable."