sumnjivo

1. suspiciously
   Neki čovek se sumnjivo mota oko zgrade. — Some man is hanging around the building suspiciously.
2. (as a predicate) it's suspicious; it's doubtful
   Sumnjivo mi je što se ne javlja. — I find it suspicious that he isn't calling.
   Sve to zvuči prilično sumnjivo. — All of that sounds pretty dubious.

This is the adverb and neuter form of the adjective sumnjiv "suspicious".