Actually, it's from the Spanish word
sierra which means "range of mountains," and is usually found in combination with other words, such as Sierra Blanca (White Range), Sierra Madre (Mother Range, or Central Range), and Nevada (Snowy Range). Occasionally
las sierras is used to designate a group of mountain ranges or ridges. In the Spanish narratives of exploration
una sierra nevada is frequently found written without capital initials, referring simply to a snow-covered range of mountains.
So for everyone who thought it was from the Bulgarian word
you were close, but it's actually Spanish.