If there are 11 people in front of you, this is an example of a time when you are the twelfth.
If a pie is cut into twelve pieces and you are given one of the pieces, this is an example of when you are given a twelfth.
Other Word Forms
Origin of twelfth
- Middle English twelfthe alteration of Old English twelfta dwo- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition