casillero
locker
casa
house
+
-illero
person or place related to
Casillero means locker and can be thought of as casa0 (house0) + -illero1 (person or place related to1), like a small house for storing things.
noun
- lockerA small compartment for storing personal belongingsGuarda tus cosas en el casillero del gimnasio. // Store your things in the gym locker.
- pigeonholeA pigeonhole or compartment for organizing documents or mailRevisa tu casillero en la oficina para ver si tienes correo. // Check your mailbox in the office to see if you have mail.