1540s, "de-horned animal," from poll (v.2) + -ard. In reference to trees cut back nearly to the trunk, from 1610s. Such trees form a dense head of spreading branches, which can be cut for basket-making, etc.
"to cut, trim, remove the top of," early 14c., pollen, "to cut short the hair" (of an animal or person), from poll (n.). Of trees or plants from mid-15c. (implied in polled), Related: Polling. A deed poll "deed executed by one party only," is from the earlier verbal meaning "cut the hair of," because the deed was cut straight rather than indented (compare indenture (n.)).
also -art, from Old French -ard, -art, from German -hard, -hart "hardy," forming the second element in many personal names, often used as an intensifier, but in Middle High German and Dutch used as a pejorative element in common nouns, and thus passing into Middle English in bastard, coward, blaffard ("one who stammers"), etc. It thus became a living element in English, as in buzzard, drunkard. The German element is from Proto-Germanic *-hart/*-hard "bold, hardy," from PIE root *kar- "hard."
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Harper, D. (n.d.). Etymology of pollard. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/pollard