Plastun or plastoon (Ukrainian, пластун) was originally a Cossack of dismounted scouting and sentry military units in Black Sea Cossack Host and later in Kuban Cossack Host in the 19th and 20th centuries. These dismounted units were introduced during the Russian-Circassian War to guard and scout beyond the "Kuban Line", a frontier in the Kuban plains, against sudden Circassian raids. The tradition of dismounted scouts, vanguard troops and ambushes, together with the term "plastuny", traces into early Cossack history of Zaporizhian Sich and mentioned, e.g., by Vladimir Dahl in his famous dictionary.
The name derives from the word plast, "sheet" via an expression "to lay like a sheet", i.e., flat and low. It its turn, the word "plastoon" gave rise to a Russian expression "ползать по-пластунски" (plastoon crawling), a way of clandestine crawling, in which the whole body rests spread on the ground and all four limbs move only sideways, in the plane, to propel the body.
Later the name "plastoon regiments" was applied to all Cossack infantry. In the Russian Imperial Army whole plastun regiments were formed. Normally Cossacks had to buy their horses and horse tack for their own money, and plastuns were relieved from these expenses. Despite this relief, regular plastun units were not very popular, since they did not fit the traditional notion of Cossack pride. Therefore plastun units tended to consist of poorer people.
The term was revived in Soviet Army during the Great Patriotic War and used in the names of several Cossack battalions and regiments. The only plastun Cossack division on that time was 9th Krasnodar Plastun Division- fought in Northern Caucasus, Poland, Chekhoslovakia and was one of the elite Soviet military units. Germans called them "Stalin's cutthroats". At the same time "plastun" (i.e., infantry) regiments existed in Cossack military that fought on the German side: in the 15th SS Cossack Cavalry Corps.