What is a anthiest?
It's nothing. However, it may be a misspelling of atheist or a
mixture of atheist and anti-theist. An atheist is convinced there's
no god. An anti-theist thinks that too, but also think that anyone
who believes in a god is preposterous.
Or perhaps you saw the adjective antsiest? Antsy - antsier -
antsiest. Least able to sit or stand still.