clean skin

/K L IY1 N S K IH0 N/
noun
  1. 1

    An unbranded animal.

  2. 2

    Someone with no prior criminal record, a person with no previous convictions; loosely, someone who has not done anything wrong before, an unblemished character.

  3. 3

    An unlabelled bottle of wine.

  4. 4

    An undercover police officer who has not done a particular task before.

  5. 5

    A cricket bat with no maker's logo

Source: https://en.wiktionary.org/wiki/clean%20skin, https://en.wiktionary.org/wiki/cleanskin