Definition of "bummer"

bummer¹

/ˈbʌ.mɚ/

noun
plural bummers
1. (obsolete, historical) A forager, especially in Sherman's March to the Sea of November to December 1864.
2. (US, slang, dated) An idle, worthless fellow, without any visible means of support; a dissipated sponger.
  Quotations
  “If I get away I sha’n’t be here,” I says, “to prove these rapscallions ain’t your uncles, and I couldn’t do it if I was here. I could swear they was beats and bummers, that’s all, though that’s worth something.
  1884 December 10, Mark Twain [pseudonym; Samuel Langhorne Clemens], chapter XXVIII, in The Adventures of Huckleberry Finn: (Tom Sawyer’s Comrade) […], London: Chatto & Windus, […]
3. A lamb (typically the smallest of a multiple birth) which has been abandoned by its mother or orphaned, and as a consequence is raised in part or in whole by humans.

bummer²

/ˈbʌ.mɚ/

noun
plural bummers
1. (colloquial) A disappointment, a pity, a shame.
  Quotations
  Looks like I'm gonna be stuck here the whole summer / Well what a bummer / I can think of a lot worse places to be
  1977, “Peaches”, in Rattus Norvegicus, performed by The Stranglers
2. (slang, dated, drug use) A psychedelic crisis; hallucinogenic drug use producing undesirable dysphoric psychological effects, most often fear, paranoia, and especially horrifying hallucinations; a bad trip.
  Quotations
  This young man never had a bummer in some 33 LSD trips. Every one of the them was a delight—everything under control. He needed only to snap his fingers, and down he came, anytime. But on Voyage 34, he finally met himself coming down an up staircase, and the encounter was crushing.
  2000, “Phase I”, in Voyage 34: The Complete Trip, performed by Porcupine Tree
interjection
1. (colloquial) Exclamation of annoyance or frustration at a bummer (disappointment).

bummer³

/ˈbʌ.mɚ/

noun
plural bummers
1. (UK, slang, derogatory, offensive) A gay man.
  Quotations
  You know, the kind of guy who would, if he wasn't gay, probably be shouting ‘Oi, gay boy!’ or ‘You bummer!’ or even ‘Oi, paki!’ or what have you. They're not enlightened.
  Matt (interviewee) quoted in 2021, Laura Dixon, Gender, Sexuality and National Identity in the Lives of British Lifestyle Migrants in Spain (page 108)

bummer⁴

/ˈbʌ.mɚ/

adjective
1. comparative form of bum: more bum