"Ardha" means "half" in Sanskrit. Since the full posture is difficult to be practiced, the half-posture, which is easier, is followed widely. This posture was first promoted by the great Yogi Matsyendranath, one of the founders of Hatha Yoga, and hence the name.
"Bhujanga" means "cobra" in Sanskrit. This asana is called "Bhujangasana" as the raised trunk, neck and head while practicing it resemble a cobra rearing its hood and about to strike, while the joined and stretched legs resemble its tail.
"Danda" means "stick" in Sanskrit.
"Janu" means "knee" and "Siras" means "head" in Sanskrit. The head is made to rest on the knee in this posture, hence the name.
"Paschima" means 'behind', 'back' or 'posterior' and 'Uttana' means to stretch out in Sanskrit. "Paschimotana" means stretching the posterior regions of the body.
"Shalabha" means "locust" in Sanskrit. The final position of this asana resembles a locust when it lowers its head to eat and raises its tail. Hence, the name.
"Sirsha" means "head" in Sanskrit. It is required to "stand on his head" in this asana and hence the name.
This pose is also called Downward Dog or Downward-Facing Dog.