Sync, short for "synchronize," is a verb for making things work together. When you lip-sync, you are moving your mouth to exactly match someone else's words spoken or sung at precisely the same time.
The verb sync, an abbreviation for "synchronize," appeared in 1929 to describe the matching of sound and picture in the new "talkies." The variation of being "in sync" appeared in 1961. The word now means any type of such matching up, including personal relationships. If you are "in sync" with another person, you are "on the same page," you get along easily. The phrase has also become popular with the growth of electronics such as computers, phones, and other communication devices that can be connected, or "synced."