Definition: a loose usually two-piece lightweight suit designed especially for sleeping or lounging
Pajamas comes from the Hindi and Urdu pājāma, which in turn comes from the Persian words for “leg” (pā) and “garment” (jāma). The “leg-garment” sense of the word’s etymology may be seen in its earliest use in English, in the beginning of the 19th century, as it first was used to simply mean “pants.” After Europeans living in Asia adopted the the loose-fitting pants, the word began to be used for clothes that one slept in.
Dear Henry: My children wear their underwear under their pajamas at night. Their grandmother says this is wrong to do.
—Life Magazine, 11 Jan., 1963