Slate’s history blog, The Vault, features a selection of “vinegar valentines,” explaining that:
Vinegar valentines were a socially sanctioned chance to criticize, reject, and insult. They were often sent without a signature, enabling the sender to speak without fear. These cards were sent not just to significant others, friends, and family, but to a larger social circle. People might post a vinegar card to a store clerk, a teacher, or a neighbor.
The tradition was quite popular. Art historian Annebella Pollen points out that these cards were often produced by the same companies that made the frilly, beautiful valentine cards adorned with hearts and flowers, but cost much less. Some historians argue that comic valentines—of which vinegar valentines were one type—made up half of all U.S. valentine sales in the middle of the nineteenth century.
Many vinegar valentines were used to enforce gender roles. Senders would use the anonymity of the card to comment on the inappropriate behavior of a couple, or the distasteful political views of a feminist friend. Women seemed to be the targets of many of the surviving examples, but balding men, pretentious artists and poets, and smelly fat guys made appearances as well.
This one from the 1870s even reinforces the idea that reading novels is a waste of time:
Memoirs of a SLACer 