"Baddha" means "locked-up" and "kona" means "angle" in Sanskrit.
"Padma" means "lotus" in Sanskrit. "Padmasana" means sitting in the "Lotus Pose". This posture is called Padmasana because the arrangement of the hands and feet resemble a lotus when seen from a distance. The two feet placed upon the opposite thigh resemble the leaves while the two hands placed one over the other resembles the lotus in full bloom.
"Sukha" means "pleasant" or "comfortable" in Sanskrit. Any cross-legged sitting position which you find comfortable, in which the body does not shake and in which you can remain at ease for a long time, is called Sukhasana.
"Supta" means "asleep" in Sanskrit. "Supta Vajrasana" is lying down supine in Vajrasana.
"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.