A bonnet was a woman's or child's hat tied under the chin and with a brim framing the face.
Yasmin Khan wore a light brown bonnet when she met Mary Shelley, Lord George Gordon Byron, John Polidori, Claire Clairmont, and Ashad on 1816. (TV: The Haunting of Villa Diodati)
Jenny Flint wore a black bonnet when she met Clara Oswin Oswald on 1892, (TV: The Snowmen) and when dealing with Winifred Gillyflower and the Crimson Horror on 1893. (TV: The Crimson Horror)