“Senioritis” (senior + -itis) is a jocular “disease” that affects many high school and college seniors. The “disease” causes the senior student to use less effort in academics or in sports. The “disease” worsens near the very end of the senior year, as the student is about to leave the institution.
 
“Senioritis” has been cited in print since at least 1904, when the term was printed in a yearbook of the University of California. “Senior slump” is a similar term for the senior year.
 
   
Wiktionary: senioritis
Noun
senioritis (uncountable)
1. (US) A tendency of seniors in high school or college to skip class or otherwise slack off due to a desire to move on, or because they have already been accepted by a college and their further academic performance is no longer as relevant.
 
Wikipedia: Senioritis
Senioritis is a colloquial term mainly used in the United States and Canada to describe the decreased motivation toward studies displayed by students who are nearing the end of their high school, college, and graduate school careers. It combines the word senior with the suffix -itis, which technically denotes inflammation but in colloquial speech is assumed to mean a general illness.
   
(Oxford English Dictionary)
senioritis, n.
U.S. humorous.
A supposed affliction of students in their final year of high school or university, esp. characterized by a decline in motivation or performance.
1907   Chicago Alumni Mag. Aug. 178/2   During the last quarter, at least, we have been suffering from an acute attack of Senioritis, a peculiar college disease to which only seniors are susceptible.
1954   Tipton (Indiana) Daily Tribune 26 Feb. 4/8   A few of the seniors are becoming afflicted with mild cases of ‘senioritis’ and, of course, they are a bit lazy and don’t want to take tests at all.
 
Google Books
The 1905 Blue and Gold of the University of California (1904), pg. 508, col. 2:
Senioritis
ETIOLOGY: An over-zealous desire to wear “Van-Dykes.”
A loss of anything else to do.
 
Google Books
August 1907, The Chicago Alumni Magazine, pg. 178, col. 2:
During the last quarter, at least, we have been suffering from an acute attack of Senioritis, a peculiar college disease to which only seniors are susceptible.
 
Google Books
February 1918, High School Life (Chicago, IL), “A Primer for High School Students” by Gilbert Strong, pg. 338, col. 1:
I would rather never get SENIORITIS, and then I would not have to have it knocked out of me.
 
23 September 1922, The Lass-o (College of Industrial Arts, Denton, TX), “A Senior Soliloquy,” pg. 3, col. 6:
Either dengue fever or Matriculation Cramp—or Senioritis.
 
13 January 1927, Syracuse (NY) Herald, “Skidding the Sport Field” with Skid, pg. 22, col. 1:
AFTER watching Syracuse defeat Cornell last night at Ithaca, the writer is convinced that the main ailment of Orange team to date is that dread malady of all college combinations: Senioritis.
   
10 December 1933, Charlotte (NC) Observer, “11 Wildcat Letter Stars Will Return,” pg. 10, col. 3:
Gene McEver, assistant coach, added, “Everything will be all right unless they get senioritis.” There will be seven seniors on the starting eleven.
 
1 November 1934, Macon (GA) Telegraph, “Lanier’s Raking Lauded by Coach,” pg. 9, col. 3:
“A fine junior team is usually a poor senior team. Vernon Smith says Georgia’s trouble is ‘Senioritis.’”
 
4 November 1954, Boston (MA) Traveler,  “BU, BC Roles Reversed,” pg. 40, col. 7:
Boston College, meanwhile, faces a Marquette team which suffered in the early part of the season with senior-itis.
 
OCLC WorldCat record
Bootstrapping “Senioritis”
Author: HARRY A GRACE; LOUIS W LEWELLYN
Edition/Format: Article Article : English
Publication: Vocational Guidance Quarterly, v7 n2 (Winter 1958): 93-96
Database: Wiley Online Library
 
OCLC WorldCat record
Independent Study Projects: An Antidote to Senioritis?
Author: Carolyn Hedges
Edition/Format: Article Article : English
Publication: The English Journal, v57 n3 (19680301): 400-401
Database: JSTOR Arts & Sciences IV Collection
 
OCLC WorldCat record
A Sure-Cure for “Senioritis”
Author: Bernard Klopp
Edition/Format: Article Article : No language available
Publication: Journal of Business Education, 46, 2, 63-64, Nov 70
Database: ERIC The ERIC database is an initiative of the U.S. Department of Education.
 
OCLC WorldCat record
An Alternative to Senioritis
Author: Virgil E Tompkins
Edition/Format: Article Article : English
Publication: The Phi Delta Kappan, v56 n9 (19750501): 625
Database: JSTOR Arts & Sciences VI Collection
 
Urban Dictionary
Senioritis
noun. A crippling disease that strikes high school seniors. Symptoms include: laziness, an over-excessive wearing of track pants, old athletic shirts, sweatpants, athletic shorts, and sweatshirts. Also features a lack of studying, repeated absences, and a generally dismissive attitude. The only known cure is a phenomenon known as Graduation.
Why didn’t study for your math test Kuhns?
Oh, who studies for a math test anyways. I got senioritis.
by Joe C. November 11, 2004
   
Kingsport (TN) Times-News
Little Miss Lifestyle: Is There a Cure for Senioritis?
April 14th, 2015 11:00 am by KARA WHITE, COMMUNITY COLUMNIST
Most people believe “senioritis” is a made-up term, coined to describe high school students who act lazy as graduation approaches. However, senioritis is much more. It is real. It is extremely serious, and very contagious. Symptoms may include, but are not limited to: a wardrobe of sweatpants, tardiness, poor grades, and a careless attitude. It affects millions of teens each year nationwide; this is not a laughing matter.