Indubitably means certainly; without doubt. It does the same job as indubitable, but it tells you how someone is speaking, not what a thing is. It is formal, and a little grand. Some people use it with a small smile, putting on a posh or old-fashioned voice — Indubitably, my dear — so it can sound serious or gently comic, depending on the tone. In everyday speech, sure, of course, and no doubt are the usual choices. Though actually, if you drop it into casual talk with a straight face, it can make you sound stiff, as if you borrowed a judge's sentence.

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