say it: ri-GRET-fuh-lee
regretfully is an adverb meaning "with regret (used in polite formulas)". It carries one recorded sense.
with regret (used in polite formulas)
“I must regretfully decline your kind invitation”
Definitions, synonyms and semantic relations for regretfully come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.