From an exchange between Alexander Neckam/Nequam and the Abbot of St. Albans. The general story goes that Alexander asked for a teaching charge at St. Albans, to which the abbot punned:
Si bonus es, venias; si nequam, nequaquam.
If you are a good man, you should come; if evil, by no means
Alexander is then said to have answered:
Si velis, veniam; sin autem, tu autem
If you wish, I shall come; if not, farewell.
(Alexander’s pun doesn’t translate well since the wit is rooted in recognizing ‘tu autem‘ as the beginning to the memento mori concluding line of medieval lectures – ‘tu autem Domine miserere nobis‘ – and hence a clever shorthand for farewell and fuck off. )
But then I have seen reversals of this sequence – that Alexander, promised the position but tired of foot-dragging, had sent his ‘reply’ first – to which the Abbott then replied with his, which then becomes the source of Alexander’s nickname rather than a pun on it.