"a pocket manual, handbook," 1620s, Latin, literally "go with me;" from imperative of vadere "to go" (see vamoose) + me "me" + cum "with."
Entries linking to vade-mecum
vamoose v.
"to decamp, be off," 1834, from Spanish vamos "let us go," from Latin vadamus, first person plural indicative or subjunctive of vadere "to go, to walk, go hastily," from PIE root *wadh- (2) "to go" (source also of Old English wadan "to go," Latin vadum "ford;" see wade (v.)).