If someone is working "flat out", it means that the person is working with the utmost haste at the maximum effort. The phrase is an English idiom used anytime someone is working so hard that he or she will not relax until the job is complete. This phrase was used in America in the early part of the 20th century, although it differs severely from an earlier form of the phrase associated with failure. There is also another use of "flat out" that occurs when someone speaks without any reservation or restraint.
People who speak English may choose to speak in a very proper manner, only using words in the most literal way possible. This might be proper, but it also might make it hard for them to communicate with people who are more familiar with a colloquial method of speaking. Instead of this rigid speaking technique, people often loosen their speech up with idioms. Idioms are phrases which can take on meanings much different from their literal definitions. The short phrase "flat out" is a popular idiom.
This phrase is most commonly used in terms of people who are working. If the phrase is used, it signifies that the person being described in this manner is giving a superlative effort. It also implies that the person is working at an extremely high rate of speed to get the job completed in as short a time as possible. For example, someone might say, "We worked flat out for two weeks to get that construction job done in time for inspection."
The odd thing about this idiom is how much it differs in meaning from an earlier usage of the same idiom. In the 19th century, someone or something described as "flat out" was considered to be a major failure. At some point, the meaning of the phrase diverged from this to the more popular meaning that was used in America in the early part of the 20th century.
Another odd thing about this idiom is that it takes on yet another distinct meaning when used in a different context. When someone uses this phrase to describe some sort of conversation, it means that the conversation takes place without any boundaries. There is a sense of candor attached to the phrase when it is used in this way. As an example, consider the sentence, "I didn't mean to hurt his feelings, but I told him flat out that he was very rude the other night."