WWTDD written on Yaz's hand. (TV: War of the Sontarans)
WWTDD was an acronym for "What Would The Doctor Do?". It was a message that Yasmin Khan secretly wrote on the palm of her hand as a reminder to emulate the Thirteenth Doctor. When Yaz met Swarm in the Temple of Atropos, he taunted her by quoting the message, which he knew about without having seeing it. (TV: War of the Sontarans)