The 27-year-old actor, who will play the Eleventh Doctor in the new series, said that a scarf he often wore led to friends giving him the nickname.
He told Esquire: "At university I had a big coloured scarf, and people would often say, 'Alright, Doctor Who?' And I thought, I rather liked the notion."
Smith also admitted that his mum encouraged him to audition for the part when she suggested he would make a good Time Lord after discovering that David Tennant would be departing the role.
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