any of numerous plants of the family Compositae and especially of the genera Carduus and Cirsium and Onopordum having prickly-edged leaves
"The thistles growing along the roadside have sharp, prickly-edged leaves that make them painful to touch."
Any of several perennial composite plants, especially of genera Cirsium, Carduus, Cynara, or Onopordum, having prickly leaves and showy flower heads with prickly bracts.
"The farmer carefully pulled up the invasive thistle from the field to prevent its prickly seeds from spreading across the meadow."
In plain English: A thistle is a prickly weed with spiky leaves and purple flowers that often grows wild in fields.
"The prickly thistles growing by the roadside made my hands bleed when I pulled them out."
Usage: The term "thistle" refers specifically to the prickly plant itself; do not use it as a verb meaning to annoy. In casual conversation, speakers often confuse the word with "tittle," which means a small mark or dot in typography.
A surname.
"The local thistle family has lived in the valley for three generations."
The word thistle traveled into Middle English from Old English, where it originally referred to the prickly plant known today. Its ultimate origin lies in a Proto-Indo-European root meaning "to prick," which is also the source of the modern English word stick.