skinny (adj.) Look up skinny at Dictionary.com
c.1400, "resembling skin," from skin (n.) + -y (2). Meaning "lean, emaciated" is recorded from c.1600. Of clothes, "tight-fitting" by 1970. In the noun sense of "the truth" it is World War II military slang, perhaps from the notion of the "naked" truth. Related: skinniness.