The characters combine to mean “to act powerful and act for one's own fortune.” It paints a picture of someone actively brandishing their power (作威) to secure their own comfort and privilege (作福), completely disregarding the well-being of those under them.
`作威作福` is deeply rooted in China's long history and Confucian ideals of governance. The ideal leader, from an emperor down to a village head, was expected to be a benevolent, paternal figure who cares for his people (为人民服务 - wéi rénmín fúwù - “to serve the people”). This idiom represents the ultimate betrayal of that ideal. It's the hallmark of the corrupt official, the cruel landlord, or the tyrannical eunuch—stock villains in Chinese history and literature. In the West, a similar concept might be a “power trip” or “lording it over someone.” However, `作威作福` carries a heavier moral weight. A person on a “power trip” might be seen as temporarily arrogant or insecure. Someone who is `作威作福` is seen as morally corrupt and failing in their fundamental duty to those they have power over. It's not just a personality flaw; it's a violation of social and ethical order. This makes it a particularly potent accusation in contexts of politics and workplace hierarchy.
This idiom is almost exclusively used in a negative and critical way. It is a formal and powerful accusation.