The purlins (highlighted in red) support the rafters in roof construction.
An example of a purlin is a horizontal wooden board holding up the rafters of a roof.
Origin of purlinMiddle English purlyn from uncertain or unknown; perhaps
Origin of purlinMiddle English
Information on the origin of the term is scant at best. According to Websters it comes from the 15th century English. According to the Oxford Concise Dictionary, it is "Middle English, perhaps of French origin". All other sources referenced Middle English or 15th Century English.