This definition is inadequate because it equates slang with colloquial style. The qualification "highly" can hardly serve as the criterion for distinguishing between colloquial style and slang.
Yet, the last line of the definition "current words in some special sense" is important and we shall have to return to this a little later.
Here is another definition of slang by the famous English writer G. K. Chesterton:
"The one stream of poetry which in constantly flowing is slang. Every day some nameless poet weaves some fairy tracery of popular language. ...All slang is metaphor, and all metaphor is poetry. ...The world of slang is a kind of topsy-turvydom of poetry, full of blue moons and white elephants, of men losing their heads, and men whose tongues run away with them — a whole chaos of fairy tales." 
The first thing that attracts attention in this enthusiastic statement is that the idioms which the author quotes have long since ceased being associated with slang: neither once in a blue moon, nor the white elephant, nor your tongue has run away with you are indicated as slang in modern dictionaries. This is not surprising, for slang words and idioms are short-lived and very soon either disappear or lose their peculiar colouring and become either colloquial or stylistically neutral lexical units.
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).
—All these meanings are certainly based on metaphor, yet they strike one as singularly unpoetical.