SSN stands for Social Security Number. It is a unique nine-digit identification number issued to U.S. citizens, permanent residents, and temporary residents. It is used primarily for identification and to track individuals' earnings and work history for Social Security benefits. The "last four" often refers to the last four digits of a person's Social Security Number.
Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: Mine is ⬛️⬛️⬛️-⬛️⬛️-⬛️⬛️⬛️⬛️. Explanations are generated by AI and may be inaccurate.