Is it ensure, insure, or assure?
"to droop, decline, tire," 1520s, apparently an alteration of flag (v.) in its sense of "droop." Transitive sense of "to make (someone or something) fatigued" is first attested 1826. Related: Fagged; fagging.
British slang for "cigarette" (originally, especially, the butt of a smoked cigarette), 1888, probably from fag-end "extreme end, loose piece" (1610s), from fag "loose piece" (late 15c.), which is perhaps related to fag (v.).
: frenetic hot-rhythm dancing, the cheap fag jokes/ like a fag partynoun
(also fagout) To fatigue; exhaust •The sense ''to study hard, go without sleep,'' is attested in Cambridge University slang by 1803: This sort of work fags me quickly (1930+)
[origin unknown; the ''homosexual'' sense may be connected with the British term fag, ''the boy servant, and inferentially the catamite, of a public-school upperclassman''; perhaps influenced by Yiddish faygele, ''homosexual,'' literally ''bird, little bird'']