Any of several thistles considered to be that used as the emblem of Scotland, variously identified as the spear thistle (Cirsium vulgare), the musk thistle (Carduus nutans), the cotton thistle (Onopordum acanthium), and other similar plants.
Origin
Mid 17th century.
Definition of Scotch thistle in US English:
Scotch thistle
noun
Any of several thistles considered to be that used as the emblem of Scotland, variously identified as the spear thistle (Cirsium vulgare), the musk thistle (Carduus nutans), the cotton thistle (Onopordum acanthium), and other similar plants.