Privacy is inextricably tied up in the Constitutional issues, so that is where is gets the most thorough examination. But there are some materials which deserve to be appended here.  The word "privacy" does not actually appear in the Constitution, but it is construed to be a broad and inalienable right just the same.  The origin of this right is grounded in tort law and a famous, oft quoted, but not so often read 1890 Harvard Law Review article by future supreme court justice Louie Brandeis called "The Right to Privacy". In it, Brandeis asserts that a person ought to be able to sue someone who violates one's right to "privacy." In a most famous passage Brandeis said:

That the individual shall have full protection in person and in property is a principle
as old as the common law; but it has been found necessary from time to time to define
anew the exact nature and extent of such protection. Political, social, and economic
changes entail the recognition of new rights, and the common law, in its eternal youth,
grows to meet the demands of society. Thus, in very early times, the law gave a remedy
only for physical interference with life and property, for trespasses vi et armis. Then the
"right to life" served only to protect the subject from battery in its various forms; liberty
meant freedom from actual restraint; and the right to property secured to the individual
his lands and his cattle. Later, there came a recognition of man's spiritual nature, of his
feelings and his intellect. Gradually, the scope of these legal rights broadened, and now
the right to life has come to mean the right to enjoy life -- the right to be let alone; the
right to liberty secures the exercise of extensive civil privileges; and the term "property"
has grown to compromise every form of possession -- intangible as well as tangible.

The courts ultimately agreed and a decade or so later a new common law cause of action was born. The term came to be used in a constitutional sense--that is, as limitation on the state's ability to interfere with the enjoyment of life or personal autonomy--some years later.

It was in Olmstead v. United States, 277 U.S. 438 (1928) (5-4) that then Justice Brandeis would apply this notion of privacy to the government in his now famous dissent in this 4th Amendment decision.