It derives from the post-classical Latin pasticium (13th century), a pie or pasty.
[1] In the 18th century, opera pasticcios were frequently made by composers such as Handel, for example Oreste (1734), Alessandro Severo (1738) and Giove in Argo (1739), as well as Gluck, and Johann Christian Bach.
The portions borrowed from other composers would be more or less freely adapted, especially in the case of arias in pasticcio operas by substituting a new text for the original one.
In late 18th-century English pasticcios, for instance by Samuel Arnold or William Shield, the "borrowed" music could be Irish or British folksongs.
Instrumental works would also sometimes be assembled from pre-existing compositions, a notable instance of this being the first four piano concertos of Wolfgang Amadeus Mozart.