It is a Castilianization of the English voice peecheewilly, a humorous and ironic neologism to call the advertising gifts delivered by pharmaceutical laboratories, especially during congresses, conventions or fairs where attendees walk the stalls looking for brochures and information about new products. It appeared for the first time in an article in the Txori-Herri Medical Journal and there may be a tribute there to the Pee-Chees folders [note: for its characteristic peach color, which in English is peach] that were used since the mid-twentieth century and served to gather loose leaves, such as informative brochures; And Willy's may be for the old meaning of "voluntary receiver of something".
It is any type of object that medical or pharmacological laboratories give to doctors to remember their products, as propaganda. They can be leveros, pens, buttons, stickers, tweezers, headphones, etc.