What is "sinestro"

Thaal Sinestro is a fictional supervillain appearing in American comic books published by DC Comics. The character was created by John Broome and Gil Kane, and first appeared in Green Lantern (vol. 2) # 7 (August 1961).

Sinestro is a former Green Lantern who was dishonorably discharged for abusing his power. He is the archenemy of the Green Lantern, though he occasionally has acted in anti-heroic roles as well. In 2009, IGN ranked Sinestro as the 15th Greatest Comic Book Villain of All Time.