means A small ornamental case, usually worn on a chain around the neck, that opens to hold a photo, a lock of hair, or some other small keepsake.

from From the Old French 'loquet,' meaning a small latch or catch — itself a diminutive of 'loc,' a lock or bolt (a cousin of the Germanic root behind English 'lock'). Originally a 'locket' was simply a little fastening or clasp; over time the word slid from the catch itself to the tiny hinged trinket that catch held shut, the small lockable thing you wore.