Planning a novvel that will feature a riverboat gambler from the mid 19th century, I started looking into what card games were commonly employed by gamblers. One of the most popular was a game called faro. In it players place bets on which of a full set of a particular suit of cards will be drawn from the card box or "faro bank". Two cards are placed on either side of the bank, one called the banker's card and the other the player's card. If a player matches the banker's card, he loses. If he matches the player's card, he wins.
I still need to learn how to play this game, how one wins and how one cheats. But one thing I learned from a video on YouTube about faro is that players would often keep track of what cards were played on small pieces of paper. These pieces of paper were called "tabs" As they were used to keep the banker honest, to "keep tabs on someone" came to mean keeping track of their actions or assertions.