(n.) the North American brown bear; (adj.) grey-haired
Grizzly bears aren’t grizzly. They might be grisly, but they’re not grizzly:
Let’s start with the second of those first. Nowadays, we tend to use the adjective grisly to mean “frightful” or “gruesome”, but its original meaning was much stronger: when it first emerged in the language in the Middle English period, it meant “horrifying” or “soul-quaking”, and typically referred to the kind of feelings or responses you might have if you witnessed something truly unnerving, or else supernatural or demonic. In that sense, it comes from an earlier verb, grise, which meant “to tremble with fright”, and probably has its origins in an ancient Germanic word meaning “shudder” or “fear”.
Despite any similarity, grizzly is unrelated. It comes from grizzle, a fourteenth century word for grey hair, which in turn has its roots in the French word for “grey”, gris.
But grizzly bears are quite literally “brown bears”, not grey—so where did that name come from?
One theory is that the name grizzly could refer to the greyish sheen the bears’ fur sometimes appears to have (in fact, silver-tip is another old name for what we’d now call a brown bear). Another theory is that it might be a figurative reference to the use of the word grizzle as another name for an old man—which might make the grizzly bear essentially “the old man of the woods”. Or else it could just be a genuine mix-up; perhaps when they were first described as such in the early 1800s, the original grizzly bears were actually intended to be grisly bears, but confusion between the two led to the creatures seemingly being misnamed.