The Spanish caja (“box”) comes from the Latin capsa for the same.
This gives us a surprising connection to some English words that, on the surface, sound very different than caja:
- Case — In the sense of, well, a box.
- Capsule — Still retains the -ps- of the original Latin.
- Cash — Originally meant “money box”. Funny how the name of the container turned into the name of the thing itself.
The Latin turned into the Spanish through an interesting pattern: the -sh- sound in Latin consistently turned into the -j- sound in Spanish (at first retaining the original pronunciation, but then under the influence of Arabic, grew to the throat-clearing sound). With caja, we have a slight variation of the pattern, where the -ps- sound turned into the -j- sound. Thus, the c-ps maps exactly to c-j.