The English idiom “dead right” simply means absolutely right, or right without a doubt. The addition of the word “dead” to the word “right” is an addition of an emphasizing word, which merely serves to underscore the idea of rightness. Using the word “dead” as an absolute can be done in several ways, all of which originated in traditional British English.
In addition to “dead right,” English speakers may also say someone is “dead on," or even "dead wrong.” Care should be taken not to confuse these phrases with another idiom, “dead to rights.” If someone says, “you got me dead to rights,” it means that the person has be caught in the act of doing something wrong, and there is no question about the person's guilt, and that the person has no "rights" or reasonable excuse by which to justify his action.
In a similar use of the word “dead” as an emphasizing word, an English speaker may ask someone “are you dead certain?” or say “I am dead certain about it.” Here, the word “dead” acts as an absolute for the word “certain.” The same idiomatic invention works with the word “sure.”
In other English-speaking societies, the above sorts of absolute phrases may not be as common as they are in traditional British English. For example, some American speakers may tend to use other phrases like “totally right,” “absolutely right,” or “completely right” rather than "dead right." Other idiomatic phrases for this idea include “spot on” and “darn tootin,’” which is a more colorful and modern idiom with an allegorical reference to “tooting” a horn.
Some other phrases for the idea of absolute rightness, besides "dead right," are more elaborate in design. An English speaker might also say, “you hit the nail on the head,” to indicate that someone is clearly right about something. Here the allegory is to carpentry, where an accurate hit on a nail with a hammer is critical to driving a nail into the wood.
Even more phrases are commonly used in English to express agreement with someone emphatically. An English speaker who is agreeing heartily with someone might say “how right you are” or “how true it is.” Someone might also say “it’s funny because it’s true,” particularly when the original statement had a humorous element. All of these serve the same purpose: to use more emphasis in agreeing, rather than just saying to someone, “you’re right.” Other types of agreement responses also utilize specific English phrases, such as the use of “you know, you’re right” along with a certain inflection, to indicate that the original speaker has just brought someone around to his or her way of thinking.
