Sinful means breaking a religious or moral law. A sinful act, sinful pride, sinful thoughts.
In religious use it is serious and precise: sin is an offence against God, and the seven deadly sins are the traditional list. In everyday use it is nearly always playful, applied to food and comfort — a sinful chocolate cake, sinfully rich — which is the commonest use of the word by a wide margin. Note sinfully; sinner; and that sinful has been in English, built this way, since Old English.
1adjectivesinfulBreaking a religious or moral law.
“The priest told them that stealing was sinful, even from the rich.”
2adjectivedeliciouslyDeliciously indulgent, in playful use.
“This chocolate cake is so rich that it's almost sinful.”
sinful is a quality of wrongdoing.
Old English synfull is sin plus -ful, and every Germanic language builds it the same way. The word has not changed shape in a thousand years.
Old English
synful — sinful, guilty, wicked, corrupt
Middle English
synful
English, today
sinful
From Middle English synful, senful, sunful, from Old English synful (“sinful, guilty, wicked, corrupt”), equivalent to sin + ful. Compare Dutch zondevol (“sinful”), German sündevoll (“sinful”), Danish syndefuld (“sinful”), Swedish syndfull (“sinful”), Icelandic syndfullur (“sinful”).
Etymology from Wiktionary, CC BY-SA