"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.
"Tri" means "three", "Kona" means "angle", and "Trikona" means "triangle" in Sanskrit The straight legs with the floor between the feet resemble the three sides of a triangle in this asana. Hence the name.
"Vajra" means "adamant" in Sanskrit. Sitting in this asana will provide firmness or stability to the sitter like that of an adamant person. Hence the name.