bur 1 also burr 
(bûr)
(bûr)n.
1.
a. A rough prickly husk or covering surrounding the seeds or fruits of plants such as the chestnut or the burdock.
b. A plant producing such husks or coverings.
2. A persistently clinging or nettlesome person or thing.
3. A rough protuberance, especially a burl on a tree.
4. Any of various rotary cutting tools designed to be attached to a drill.
[Middle English burre, of Scandinavian origin.]