E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Q in a corner. Something not seen at first, but subsequently brought to notice. The thong to which seals are attached in legal documents is in French called the queue; thus we have lettres scellées sur simple queue or sur double queue, according to whether they bear one or two seals. In documents where the seal is attached to the deed itself, the corner where the seal is placed is called the queue, and when the document is swornto the finger is laid on the queue.
In a merry Q (cue). Humour, temper; thus Shakespeare says, My cue is villanous melancholy (King Lear, i. 2).
Old Q. The fifth Earl of March, afterwards Duke of Queensberry.