Definition
"Doubtlessly" is essentially synonymous with "doubtless" and means without doubt; certainly; very likely. It implies a strong belief or expectation, though perhaps with a slightly less formal tone than "undoubtedly." Think of it like being pretty sure your favorite team will win π. It conveys a high probability. This contrasts with uncertainty. For instance, 'Doubtlessly, she will enjoy the party' means you anticipate she will have fun. It's a close cousin to "doubtless." π