Thomas Jefferson famously used the phrase "wolf by the ear" to describe the country's relationship with slavery in an April 22, 1820 letter to Massachusetts Congressman John Holmes (paragraph breaks and emphasis added):
I thank you, Dear Sir, for the copy you have been so kind as to send me of the letter to your constituents on the Missouri question. it is a perfect justification to them. I had for a long time ceased to read the newspapers or pay any attention to public affairs, confident they were in good hands, and content to be a passenger in our bark to the shore from which I am not distant.
but this momentous question, like a fire bell in the night, awakened and filled me with terror. I considered it at once as the knell of the Union. it is hushed indeed for the moment. but this is a reprieve only, not a final sentence. a geographical line, coinciding with a marked principle, moral and political, once concieved [sic] and held up to the angry passions of men, will never be obliterated; and every new irritation will mark it deeper and deeper.
I can say with conscious truth that there is not a man on earth who would sacrifice more than I would, to relieve us from this heavy reproach, in any practicable way. the cession of that kind of property, for so it is misnamed, is a bagatelle which would not cost me in a second thought, if, in that way, a general emancipation and expatriation could be effected: and, gradually, and with due sacrifices, I think it might be. but, as it is, we have the wolf by the ear, and we can neither hold him, nor safely let him go. justice is in one scale, and self-preservation in the other.
of one thing I am certain, that as the passage of slaves from one state to another would not make a slave of a single human being who would not be so without it, so their diffusion over a greater surface would make them individually happier and proportionally facilitate the accomplishment of their emancipation, by dividing the burthen on a greater number of co-adjutors. an abstinence too from this act of power would remove the jealousy excited by the undertaking of Congress, to regulate the condition of the different descriptions of men composing a state. this certainly is the exclusive right of every state, which nothing in the constitution has taken from them and given to the general government. could congress, for example say that the Non-freemen of Connecticut, shall be freemen, or that they shall not emigrate into any other state?
Jefferson used the same phrase - "wolf by the ear" - also referring to slavery, in a July 18, 1824 letter to Lydia Huntley Sigourney (emphasis added):
I am not apt to despairing, yet I see not how we are to disengage ourself from that deplorable entanglement, we have the wolf by the ear & feel the danger of holding or letting loose. . . . I shall not live to see it but those who come after us will be wiser than we are, for light is spreading and man improving. To that advancement I look, and to the dispensations of an all-wise and all-powerful providence to devise the means of effecting what is right.
Isn't the use of the singular "ear" like, totally weird? Would you ever hold a wolf by one ear? Of course not. The wolf is lunging for your throat. You're desperately holding him (or her, I suppose) by both ears.
It appears that the Sage of Monticello in his dotage was misremembering his classical studies. The phrase was already a commonplace expression back in 161 BC, when the Roman comic playwright Terence used it in his play Phormio (Act 3, Scene 2). The phrase involved, as you might expect, multiple ears :
O most fortunate Antipho!
To have in your possession the object of your love, and have no occasion to encounter such a nuisance as this.
What I, in my possession? Why yes, as the saying is, I've got a wolf by the ears; for I neither know how to get rid of her, nor yet how to keep her.
I've checked the Latin text, and both ears are there ("auribu[s]" is plural):
mihin domist? immo, id quod aiunt, auribu' teneo lupum; nam neque quo pacto a me amittam neque uti retineam scio.
Roughly 200 years later, the phrase was a favorite saying of the Roman emperor Tiberius. Suetonius reports in his biography of Tiberius that he often used the phrase to describe the danger experienced in ruling the empire: you either were the ruler, or you were dead:
The cause of [Tiberius'] hesitation [in becoming emperor] was fear of the dangers which threatened him on every hand, and often led him to say that he was "holding a wolf by the ears."
As before I've checked the Latin text, and those multiple ears ("auribus") are still there:
Cunctandi causa erat metus undique imminentium discriminum, ut saepe lupum se auribus tenere diceret.