Triskaidekaphobia

Triskaidekaphobia was, as stated by Nardole, a fear of the number 13. The Twelfth Doctor assured his companion Bill Potts that the fear was irrational. (COMIC: A Confusion of Angels)

Behind the scenes
The reference towards the "fear of 13", although relevant to the story itself, is additionally also a joke about the controversies surrounding Jodie Whittaker being cast as the Thirteenth Doctor. The dialogue was featured in the final issue (Year 3 #13) of Titan's ongoing Twelfth Doctor series, likely explaining the inclusion.