"Fit for a king" is an English idiom that is used to describe anything that is of exceptional quality. The phrase gets its meaning from the fact that anything that is considered suitable for a king to receive must be without any noticeable flaws. This idiom originated sometime around the 18th century when many countries' governments were still ruled by royal families. In its modern derivation, the phrase "fit for a king" often is used to describe meals, perhaps because kings were often the recipients of lavish feasts in their honor.
Using an idiom when speaking is a way for someone to add color and expressiveness to what otherwise might be a boring description. These idioms take their meaning not so much from the literal meaning for their words but rather from the usage that has come to be accepted in the culture. The figurative meanings may have originated from literal sayings, but they usually evolve to mean something quite different. One idiom that has survived for hundreds of years is the expression "fit for a king."
Quite simply, anything that is described in this manner must be truly outstanding or exceptional. As an example, a particularly sumptuous meal might be described in this way. Someone might say, "I really have to say that the meal I ate at that restaurant was fit for a king." There is a bit of exaggeration involved here, since the restaurant's meal likely couldn't match up to a typical king's feast. Such exaggeration gives this idiomatic expression its colorful mature.
Meals might be the most common instances of this phrase being used, but it can actually be utilized to describe any number of extraordinary occurrences. For example, consider the sentence, "That is one stunning bed; I'd say it was fit for a king." The phrase often connotes a certain opulence that common people often associate with royalty. As such, the phrase can depict anything that ascends to such glorified heights in the minds of those doing the describing.
The phrase gets its meaning and its origin from the fact that kings usually received the best of everything while they ruled. While royal families rule less frequently in modern times — and often only have symbolic power — the aura that surrounds such royalty is still felt very strongly in society. As long as this is the case, it's likely the phrase will never go out of fashion.