Jocular, humorous term for a surgical face mask, literally 'face pants'. Used playfully, often in casual conversation or online.
During hay fever season, I can't go a day without my face mask.
More people are wearing face masks on the train these days, huh.
Compound of 顔 (kao, 'face') and パンツ (pantsu, 'pants, underwear'), humorously likening a face mask to underwear for the face.