It may be a pun. Looking up [magma](http://fr.wikipedia.org/wiki/Magma_(alg%C3%A8bre)#cite_note-0) in the French wikipedia<sup>*</sup>, another name for *magma* in French is *groupoïde de Ore*<sup>&dagger;</sup>. Here Ore is a [Norwegian mathematician](http://en.wikipedia.org/wiki/%C3%98ystein_Ore), but *ore* in English is mineral-bearing rock, whereas *magma* (in both English and French) is molten rock. 

Would Bourbaki have based a mathematical term on this pun? I'm not in a good position to judge; maybe somebody else could comment on this.


<sup>*<sup> Since Bourbaki was a pen name used by a group of French mathematicians, this is the right language to search in.  
<sub><sup>&dagger;</sup> There is also a [*groupoïde de Brandt*](http://en.wikipedia.org/wiki/Groupoid), also called a *groupoid* in English, which would explain why Bourbaki felt compelled to coin a new name.</sub>