Two possible etymologies emerge for this poorly attested expression. The first would have the term be a derivation from the CL placere. The compound ‘lettre de placet’ is attested in both Gdf [‘lettre de plachet’ 1365] and the DMF [‘lettre de placet’ 1488] with the sense of a letter addressed to a king.

It is also possible to consider the term a derivative of the Germanic etymon *placken 'to build a wall'. The locution ‘lettre de placard’ is attested in the DMF from 1460 in the sense of a letter that is under a seal (not folded). The OED believes this to be the etymology for the related terms of ‘letter of placard’ (1482), as well as placate and placket in the sense of ‘an official or public document’.

The term is only attested in this single use. The variant reading found in Cal Rel Scot, the result of a different base manuscript, is difficult to explain, as the verb placketter is not otherwise attested during this period, though the verb to placard is attested in English in the 18th century.