What is the origin of vade mecum?
A vade mecum in English is something, especially a book or manual, that a person carries about for consulting. The English phrase comes from the Latin phrase vāde mēcum “go with me.” The first word, vāde, is the second person singular imperative of vādere “to go, advance, proceed,” from the same Proto-Indo-European root wadh– “to go” as the Germanic (English) wade. Mēcum ”with me,” and its kindred forms tēcum “with thee,” nōbiscum “with us,” and vōbiscum “with you,” are relics or fossils in Latin of an earlier stage in the language when “prepositions” (elements that precede the words governed) were “postpositions” (the elements followed the words governed). During imperial times, the anomalous mēcum and tēcum were strengthened, reinforced by the “regular” preposition cum, yielding cum mēcum and cum tēcum, which persist in modern Spanish as conmigo and contigo. Vade mecum entered English in the 17th century.