grdno

1. (of something unpleasant) badly; terribly; sorely
   Ako to misliš, grdno se varaš. — If you think that, you're sorely mistaken.
   Grdno smo se namučili sa tim papirima. — We had a terrible time with those papers.

Most often in grdno se prevariti / pogrešiti "to be badly mistaken".