expressions that are not considered standard in the speaker's
language or dialect but are considered acceptable in certain social settings. Slang expressions may act as euphemisms and may be used as a means of identifying with one's peers.As to the author's words "all slang is metaphor", it is a true observation, though the second part of the statement "all metaphor is poetry" is difficult to accept, especially if we consider the following examples: mug (for face), saucers, blinkers (for eyes), trap (for mouth, e. g. Keep your trap shut), dogs (for feet), to leg (it) (for to walk).
The definition of Slang