Thistly is from thistle (Old English thistel) + the adjectival suffix -y, from Proto-Germanic *thistilaz. Important recorded forms include thistel (Old English) and *thistilaz (Proto-Germanic). Morphologically, the -ly ending turns the base adjective or participial form into an adverbial manner word, or preserves an older adjectival pattern where the part of speech allows it. The modern sense continues the older lexical line without requiring a separate origin. In current use it covers full of or resembling thistles; prickly. The wider family includes related words such as thistle and thistledown.