Indubitably means "without doubt." If you say that you are indubitably going to run for class president, you're sure of it.
With five syllables, indubitably is kind of a mouthful. Here's how to say it: "in-DOO-bit-a-blee." And while it is more of an old-fashioned adverb, indubitably remains a unique way to say "undoubtedly." In fact, doubt and indubitably share a root in the Latin word dubitabilis, meaning "doubtful." Adding the prefix in- makes something done indubitably without a doubt.