Young Brunette Girl With Pigtails Undresses Her Blue Chemise On The Bank Of The River

From the later 17th century through the 19th century, the term came to be applied to any braided (plaited, in British parlance) hairstyle. The British army also adopted a single pigtail or "queue" as its standard dress for long hair.
Robert Louis Stevenson mentions "pigtail" referring to hair and then to "pigtail tobacco" in the first and fourth chapters of Treasure Island, respectively.
Most dictionaries still define "pigtail" as a single tight braid. However, many American English speakers use the term to describe two symmetrical bunches of hair on either side of the head, braided or not. In some cases, the term only applies to unbraided hair. This usage of the term can be seen on personal and professional websites devoted to hairstyles or even by typing "pigtails" into a search engine.