Vinegar valentines are greeting cards, or, rather, insult cards, that come in the form of an insult, decorated with a caricature and, below that, an insulting poem. Ostensibly given on Valentine's Day

Valentine's Day

Saint Valentine's Day, commonly shortened to Valentine's Day, is an annual commemoration held on February 14 celebrating love and affection between intimate companions. The day is named after one or more early Christian martyrs named Saint Valentine, and was established by Pope Gelasius I in 496...

, the caricature and poem is about the "type" that the recipient belongs to--spinster

Spinster

A spinster, or old maid, is an older, childless woman who has never been married.For a woman to be identified as a spinster, age is critical...

, floozy, dude

Dude

A dude is an individual, typically male. The female equivalent, which is used less often, is "dudette" or "dudess". However, "dude" has evolved to become more unisex to encompass all genders, and this was true even in the 1950s....

, scholar, etc.

The cards are usually simply a sheet of thin, colored paper, about the size of a modern greeting card. They were later also produced in the form of postcards. They were usually sent anonymously. Postmasters sometimes confiscated these cards as unfit to be mailed.

The cards were first produced in the late Victorian era

Victorian era

The Victorian era of British history was the period of Queen Victoria's reign from 20 June 1837 until her death on 22 January 1901. It was a long period of peace, prosperity, refined sensibilities and national self-confidence...

 and enjoyed their greatest popularity in that period and in the first quarter of the 20th century.

Some people mistakenly call them penny dreadfuls

Penny Dreadful

A penny dreadful was a type of British fiction publication in the 19th century that usually featured lurid serial stories appearing in parts over a number of weeks, each part costing an penny...

, although that term in fact refers to a form of potboiler

Potboiler

Potboiler or pot-boiler is a term used to describe a poor quality novel, play, opera, or film, or other creative work that was created quickly to make money to pay for the creator's daily expenses . Authors who create potboiler novels or screenplays are sometimes called hack writers...

 fiction.

One pop culture reference to vinegar valentines is found in Bill Watterson

Bill Watterson

William Boyd Watterson II , known as Bill Watterson, is an American cartoonist and the author of the comic strip Calvin and Hobbes...

's Calvin and Hobbes

Calvin and Hobbes

Calvin and Hobbes is a syndicated daily comic strip that was written and illustrated by American cartoonist Bill Watterson, and syndicated from November 18, 1985, to December 31, 1995. It follows the humorous antics of Calvin, a precocious and adventurous six-year-old boy, and Hobbes, his...

 comic strip. Calvin often gives Susie Derkins vinegar valentines so as to hide his true affections for her under a veil of immature distaste.