Henry Bradley writes that "Slang sets things in their proper place with a smile. So, to call a hat 'a lid' and a head 'a nut' is amusing because it puts a hat and a pot-lid in the same class".  And, we should add, a head and a nut in the same class too.
"With a smile" is true. Probably "grin" would be a more suitable word. Indeed, a prominent linguist observed that if colloquialisms can be said to be wearing dressing-gowns and slippers, slang is wearing a perpetual foolish grin. The world of slang is inhabited by odd creatures indeed: not by men, but by guys (R. чучела) and blighters or rotters with nuts for heads, mugs for faces, flippers for hands.
All or most slang words are current words whose meanings have been metaphorically shifted. Each slang metaphor is rooted in a joke, but not in a kind or amusing joke. This is the criterion for distinguishing slang from colloquialisms: most slang words are metaphors and jocular, often with a coarse, mocking, cynical colouring.
This is one of the common objections against slang: a person using a lot of slang seems to be sneering and jeering at everything under the sun. This objection is psychological. There are also linguistic ones.
G. H. McKnight notes that "originating as slang expressions often do, in an insensibility to the meaning of legitimate words, the use of slang checks an acquisition of a command over recognised modes of expression ... and must result in atrophy of the faculty of using language". 
H. W. Fowler states that "as style is the great antiseptic, so slang is the great corrupting matter, it is perishable, and infects what is round it". 
McKnight also notes that "no one capable of good speaking
or good writing is likely to be harmed by the occasional employment of slang, provided that he is conscious of the fact..."