mid-15c., abbreviation of master (n.); also see mister. Used from 1814 with a following noun or adjective, to denote "the exemplar or embodiment of that quality," as in Mr. Right "the only man a woman wishes to marry" (1826); Mr. Fix-It "one skilled in setting right difficult situations" (1912); Mr. Big "head of an organization; important man" (1940). The Procter & Gamble advertising character Mr. Clean dates from 1959. The plural Messrs. (1779) is an abbreviation of French messieurs, plural of monsieur, used in English to supply the plural of Mr., which is lacking.
1836, "of or relating to Mr. Pickwick," the hero of Dickens' "Pickwick Papers."
"characterized by an ostentatious hypocritical display of benevolence or high principle," 1851, after Mr. Pecksniff, character in Dickens' "Martin Chuzzlewit" (1844).
[Coleridge] evades the only charge brought against [Southey], by repelling one not brought against him, except by his Antijacobin patrons—and answers for his friend, as if he was playing at cross-purposes. Some people say, that Mr Southey has deserted the cause of liberty : Mr Coleridge tells us, that he has not separated from his wife. They say, that he has changed his opinions : Mr Coleridge says, that he keeps his appointments ; and has even invented a new word, reliability, to express his exemplariness in this particular. [from a review of "Biographia Literaria" in Edinburgh Review, August 1817]
also Mr. Chad, simple graffiti drawing of a head peering over a fence or wall, with the caption, "Wot, no ______?" (the U.S. version usually had "Kilroy was here"), 1945, British, of unknown origin, a reaction to war-time shortages and rationing.
late 14c., "deadly, destructive to life; causing or threatening death" (of illness, poisons, wounds, etc.); also, of persons or the body, "doomed to die, subject to death;" from Old French mortel "destined to die; deserving of death" and directly from Latin mortalis "subject to death, mortal, of a mortal, human," from mors (genitive mortis) "death."
This is reconstructed to be from PIE *mr-o- "to die," *mr-to- "dead," *mr-ti- "death," all from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death). The most widespread Indo-European root for "to die," it forms the common word for it except in Greek and Germanic.
"Subject to death," hence "human, of or pertaining to humans" (early 15c.). Also from late 14c. as "implacable, to be satisfied only by death" (of hatreds, enemies, etc.). Meaning "extreme, very great" is from late 14c. A mortal sin (early 15c., opposed to venial) is one that incurs the penalty of spiritual death.
in reference to opposite aspects of a person's character is a reference to Robert Louis Stevenson's story, "The Strange Case of Dr. Jekyll and Mr. Hyde," published in 1886. Jekyll, the surname of the respectful and benevolent man, is of Breton origin and was originally a personal name. Hyde in reference to the dark, opposite side of one's personality is from 1887.
"Though so profound a double-dealer, I was in no sense a hypocrite. Both sides of me were in dead earnest; I was no more myself when I laid aside restraint and plunged in shame, than when I labored, in the eye of day, at the furtherance of knowledge or the relief of sorrow and suffering." [Robert Louis Stevenson, "The Strange Case of Dr. Jekyll and Mr. Hyde," 1886]