scoring combination of January, December, and August 20-point cards
A specific hanafuda yaku (scoring combination) consisting of the Pine (January), Paulownia (December), and Susuki (August) 20-point cards. Also known as 出来役 (dekiyaku).
See also: 出来役
Matsukiribōzu is a yaku formed by the three cards: Pine, Paulownia, and the Monk (August 20-point card).
出来役 is a broader term for scoring combinations in hanafuda, while 松桐坊主 is one specific combination.
The name combines the kanji for the three cards involved: 松 (pine, January), 桐 (paulownia, December), and 坊主 (monk, referring to the August 20-point card which depicts a monk under the moon).