1. A tree that has had its top and branches cut off, to encourage dense new growth in the form of a crown of branches.
2. An animal that has shed its horn or antlers or had them removed.
To convert into a pollard; to poll.
Etymology: from German pol, head or top. "Poll tax" has the same etymology.
The animal-related definition appears to be archaic, although "polled" is in use. For example, "polled Hereford" (a type of Hereford genetically bred to not have horns).
(My housemate, having seen Guided by Voices in concert, recommended "drank all the beer" as an alternate definition of "pollard".)