Note to Twelfth Night, 1.3.99, "it will not curl by nature"
This is the punch-line of Sir Toby's jest. Sir Andrew says that he wished he had devoted his time to "the arts" (languages, music, astronomy, etc.) In answer, Sir Toby says that if Sir Andrew had devoted his time to "the arts," he would have "an excellent head of hair," which sounds totally illogical. But Sir Toby uses "arts" to mean "artifices" (such as artificially curled hair), sosince Sir Andrew's hair "will not curl by nature""arts" or artifices will indeed give him an "an excellent head of hair."
Sir Andrew Aguecheek