What is the meaning of Bummer?

A forager, especially in Sherman's March to the Sea of November to December 1864.

An idle, worthless fellow, without any visible means of support; a dissipated sponger.

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.

A disappointment, a pity, a shame.

A psychedelic crisis; hallucinogenic drug use producing undesirable dysphoric psychological effects, most often fear, paranoia, and especially horrifying hallucinations; a bad trip.

Exclamation of annoyance or frustration at a bummer (disappointment).

A gay man.

comparative form of bum: more bum

to bum (ask for something for free)

Source: wiktionary.org