perpendicular pronoun

Sir Humphrey: The identity of the official whose alleged responsibility for this hypothetical oversight has been the subject of recent discussion is not shrouded in quite such impenetrable obscurity as certain previous disclosures may have led you to assume; but not to put too fine a point on it, the individual in question is, it may surprise you to learn, one whom your present interlocutor is in the habit of defining by means of the perpendicular pronoun.

Hacker: I beg your pardon?

Sir Humphrey: It was... I.