nezamislivo

1. unimaginably; inconceivably
   Stanovi u centru su nezamislivo skupi. — Flats in the centre are unimaginably expensive.
2. (as predicate) unthinkable; inconceivable
   To je pre dvadeset godina bilo nezamislivo. — Twenty years ago that was unthinkable.
   Nezamislivo je da ga niko nije video. — It's inconceivable that nobody saw him.

Often used in the construction nezamislivo je da… "it is unthinkable that…".