The word "poacher" derives from the very "to poach", which has two possible origins for its illegal hunting meaning: one theory connects it to the Old French "pochier" (to poke or thrust), perhaps suggesting pushing into someone else's territory.  It could also be referring to thrusting into game with a spear or stick, in order to kill it.  Another suggests it comes from the Middle English "pocchen" (to bag), referring to putting game into a "pouch", eferring to hiding game after it was illegally killed. This is supported by the term "poacher's pocket," a type of bag used for concealing game, and the slang term "pocket" for stealing small items.  

According to the Oxford English Dictionary (OED), the earliest known use of the noun "poacher" (in the sense of someone who illegally hunts game) in English is from 1574.  The word first appeared in John Studley's translation of the *Pageant of Popes*. The OED entry specifies two different nouns for "poacher," with a second one for an "egg-poacher" dating to 1861.  The original noun, referring to someone who poaches game, is the one from 1574 that was initially used to describe someone who intruded or encroached upon the property or rights of another.  The sense of a person who illegally hunts game on another's land became common in the late 1600s.