COMMONPLACE, a translation of the Gr., i.e. a passage or argument appropriate to several cases; a "common-place book" is a collection of such passages or quotations arranged for reference under general heads either alphabetically or on some method of classification. To such a book the name adversaria was given, which is an adaptation of the Latin adversaria scripta, notes written on one side, the side opposite (adversus), of a paper or book. From its original meaning the word came to be used as meaning something hackneyed, a platitude or truism, and so, as an adjective, equivalent to trivial or ordinary. It was first spelled as two words, then with a hyphen, and so still in the sense of a "common-place book."
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)