»miching mallecho

The Night Writer has the best explanation of the curious phrase "miching mallecho": it means mischief.

As the play within the play begins in Shakespeare's Hamlet (Act III, Scene 2) and the players act out the poisoning of the king and the wooing and winning of the queen by the poisoner, Ophelia enters and cries, "What means this, my lord?" and Hamlet answers, "Marry, this is miching mallecho; it means mischief. Thus Shakespeare himself supplies the definiition (sic): mischief. Mallecho was derived from the Spanish noun malhecho (evil deed), base on the prefix mal-(evil) plus hecho (deed). Miching (MICH ing) is an adjective made of the present participle of the verb miche, meaning to "skulk" or "slink," thought to be a variant of mooch (British slang for "slouch about" or "skulk," differing from the American slang usage, to "scrounge," both, however, coming from the same source, Middle English michen, to skulk or hide)...

Thus, miching mallecho means "sneaky mischief." You may never run into this eloquent phrase in contemporary literature, unless you happen to read An Awkward Lie by the English whodunitist Michael Innes (b. 1906), where his detective Sir John Appleby, considering the mysterious disappearance of a corpse from a golf bunker, wonders about this "elaborate piece of miching malicho." Malicho is a variant of mallecho, or vice versa. Some authorities say that it is vice versa, mallecho, influenced by the Spanish, being a learned emendation of malicho, the form favored by Michael Innes.

I know it not from Innes, but from Dorothy Sayers, who uses it for a provocative heading in Unnatural Death.