means Excellent or outstanding — so good it deserves comparison to the stars; literally, of or relating to stars.

from From Latin stellaris, 'pertaining to a star,' built on stella, 'star.' That stella is an old, far-reaching root — a likely cousin of the Greek aster (which gives us asterisk and astronomy) and even of English star itself, all possibly tracing back to a shared ancestor in the prehistoric parent tongue. The figurative leap from 'astronomical' to 'fantastic' is fairly modern: people simply found that pointing at the heavens was the highest praise available.