jamčiti

1. Croat. to guarantee; to vouch for
   Jamčim vam da će sve biti gotovo sutra. — I guarantee you everything will be done tomorrow.
   Za njega jamčim svojom glavom. — I vouch for him with my life.
2. Croat. (of a law, document) to guarantee; to ensure
   Ustav jamči slobodu govora. — The constitution guarantees freedom of speech.

In Serbia: garantovati or jemčiti; jamčiti is the Croatian form. To vouch for someone is jemčiti (jamčiti) za + acc.