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The "thin blue line" is a term that typically refers to the concept of the police as the line between law-and-order and chaos in society. The "blue" in "thin blue line" refers to the blue color of the uniforms of many police departments. (Wikipedia)

The phrase originated as an allusion to The Thin Red Line incident during the Crimean War in 1854, wherein a Scottish regiment—wearing red uniforms—famously held off an Imperial Russian Army cavalry charge. Its use referring specifically to the police was popularized by Los Angeles Police Department Chief William H. Parker during the 1950s; author and police officer Joseph Wambaugh in the 1970s, by which time "thin blue line" was used across the United States; and Errol Morris's documentary The Thin Blue Line (1988). In recent years, the symbol has also been used by the Blue Lives Matter movement in the United States, which aims to show solidarity with the police, and a number of far-right movements in the US, particularly after the Unite the Right rally in 2017. (Wikipedia)

The term is adapted from The Thin Red Line, an incident of the 1854 Battle of Balaclava during the Crimean War where troops the 93rd Regiment of Foot successfully stood their ground against a Imperial Russian Army cavalry charge. This action was widely publicized by the British press and depicted in several artworks, becoming one of the most famous episodes of the entire conflict. The name is now used for firefighters today.  In the book Lawtalk, James Clapp and Elizabeth Thornburg say the term spread to other professions, e.g., a "thin white line of bishops". An early known use of the phrase "thin blue line" is from a 1911 poem by Nels Dickmann Anderson, titled "The Thin Blue Line". In the poem, the phrase is used to refer to the United States Army, alluding both to the Thin Red Line and to the fact that US Army soldiers wore blue uniforms from the eighteenth century through the nineteenth century.   (Wikipedia)

It is unknown when the term was first used to refer to police. New York police commissioner Richard Enright used the phrase in 1922. In the 1950s, Los Angeles Police Chief Bill Parker often used the term in speeches, and he also lent the phrase to the department-produced television show The Thin Blue Line. Parker used the term "thin blue line" to further reinforce the role of the LAPD. As Parker explained, the thin blue line, representing the LAPD, was the barrier between law and order and social and civil anarchy. (Wikipedia)