Pajamas are clothes made for sleeping in. When you're not feeling well, there's nothing better than putting on your pajamas, climbing into bed, and having someone bring you a cup of tea.
Pajamas is always plural — it's got that in common with words like pants, shorts, and trousers. Like these other plural clothing words, pajamas have two legs — they're like a loose, soft, and very comfortable pair of pants. This word started out in 1800 as two: pai jamahs, "loose trousers tied at the waist," worn by Europeans in imitation of traditional Indian Muslim clothing. The Persian root, paejamah, combines pae, "leg," and jamah, "clothing."