It is the deliberate manufacture of a reputation.

You do not accuse your target directly, at first; that invites scrutiny.

Instead you seed doubt, quietly and repeatedly, in small deniable increments, a lost item here, a vague complaint there, always with a reasonable face on it, until the people around your target have absorbed, without quite noticing, the impression that this is someone not entirely to be trusted.

By the time you strike, you are not introducing a new idea.