Visiting a sick or injured relative is commonly accepted as a valid excuse to miss an event. If it's a close enough relative, you might even get out of work for that hospital visit. But who says you're really headed to the hospital?
So this trope comes into play when a character makes up a sick or invalid relative in order to escape their daily life/obligations, excusing their absence to others as a visit to said relative. Common examples include a student missing on lessons and exams, a worker taking a day off, or a rich aristocrat/Millionaire Playboy going out to town or to the country.
This can be played several ways. The "visit" could be an alibi to let that character visit someone, or an excuse to get out of sight before the full moon rises. Or, most likely, the character just can't face going out to dinner with the Obnoxious In-Laws, and invents a cousin on the other side of the family to explain his absence.
- In Harry Potter and the Prisoner of Azkaban, Lupin mentions that he would often claim his mother was chronically ill as an excuse for his frequent absences, since he didn't want anyone finding out he was a werewolf and needed to be away from the school during his monthly transformations.
- In later Discworld watch books, the City Watch gives an allowance of days off for three grandmother's funerals per year. This was instituted after Nobby started taking up to eight.
- In Mercedes Lackey's Elizabethan novels, Rhoslyn excuses her regular absences from attending on Mary Tudor by saying her brother has a chronic illness. (The brother is real, the illness isn't.)
- In Belisarius Series the hunted runaway princess Shakuntala escaped the evil empire's city by dressing in the finery of-well-a princess-but a generic one rather then the one that they were looking for (to make sure anyone would be too intimidated by her awesome authority to question), and traveling in an ostentatiously princessly caravan to what she claimed was a dying father. Presumably that was felt needed to explain why a woman would be traveling in early medieval India.
- Castle: When Castle's ditzy first ex-wife Meredith comes back into town, she at one point tells their daughter Alexis' high school that her grandmother has died so can she please be excused from classes. She neglects to mention that said grandmother died several years ago; she really just wanted to take Alexis shopping.
- In Brit Com The Brittas Empire, Helen Brittas, the semi-sane and barely functioning wife of the idiot Gordon, uses this excuse to get away from the husband who is driving her mad. she claims she is going to nurse an invalid elderly uncle for two days, but as she is seen in stockings and suspenders and dressing to kill, the audience is primed to doubt this... she does pack a "nurse's outfit", though.
- On The Big Bang Theory, Leonard makes up a story about going to a symposium to keep from going to Penny's recital. Sheldon feels that story is insufficient and makes up a more elaborate one that involves going out of state to visit a cousin in rehab, even going as far as hiring an actor to play said cousin.
- On M*A*S*H Klinger tries many times to get out of the army, including presenting the Colonel with a letter from home saying his mother is dying. The Colonel pulls out Klinger's file filled with letters from home saying his mother is dying, father is dying, sister is dying, sister is pregnant, sister is dying and mother pregnant, etc.
- On Law & Order, Ed Green and Cyrus Lupo are in the middle of an investigation that Green discovers to be tied to an old flame. He tries to get out to warn her by leaving work early, claiming that he needs to visit a sick friend. And thus starts a chain of events that end up costing him his career...
- Discussed in Frasier ("The Two Mrs Cranes"). Gil is looking to get out of a dinner party, and asks Frasier to back him up on an excuse. Frasier explains he's driving his father to his army reunion. Gil assumes it's this trope, and laments that he can't use the same excuse - he already "killed off" his father to escape a "Labour Day Clam Bake".
- This trope was codified, if not made by Oscar Wilde's 1895 comedic masterpiece The Importance of Being Earnest, in which one of the main characters, Algernon, admits to inventing an entirely fictional invalid friend going by the name of 'Bunbury' whom he "visits" whenever "Bunbury" is "sick"; actually as an excuse to leave town and set off to do whatever he may please. A similar scheme is also used by his friend who goes by the name "Ernest", (actually one Jack Worthing) which, under similar circumstances to Algernon's "Bunbury", is a character Jack has invented to allow himself the excuse to visit town from his country home. When in town, he adopts the persona of Ernest so as to keep up appearances.
- In an episode of The Fairly OddParents, Timmy's mother calls in saying that she has to visit a sick aunt, whose name she invents on the spot. She does this along with the mothers of Timmy's other friends so they can go on a relaxing ski retreat alone.
- In an episode of Family Guy, Meg knocks on the door of the boy she was going to go to the prom with. He answers, then goes back into the house for a minute. We hear a gunshot, and he comes back out crying that he can't go because he must attend his brother's funeral. He didn't "make up" his brother's condition, but presumably created it by shooting him—so this could be the trope taken up to a pretty twisted eleven.
- In The Simpsons episode Homer's Phobia, Smithers spots Camp Gay antiques dealer John with the title family at a restaurant and confronts him with, "So, this is your 'sick mother'?"