‘Even now, now, very now, an old black ram Is topping your white ewe.’

Iago is telling Brabantio, in the crudest terms available, that his daughter is in bed with Othello. The ‘old black ram’ is Othello; the ‘white ewe’ is Desdemona. Topping (or tupping) is a farming word for a ram mating with a ewe.

Every word is chosen to enrage a sleeping old man. It reduces a marriage to livestock breeding, and it leads with race: black against white, beast against innocent. ‘The devil will make a grandsire of you’ twists the knife. Grandsire means grandfather, so Iago is jeering that Brabantio’s grandchildren will be fathered by a man he calls a devil.

It works. Brabantio searches the house, raises armed men, and by the next scene is accusing Othello of witchcraft to his face: ‘thou hast enchanted her.’

These are almost the first words the play gives anyone about Othello, shouted up at a window in the dark. Iago will not even use his name. So the audience meets a racist caricature before it meets the man, and the calm, commanding Othello of the next scene proves the caricature a lie. That gap is deliberate.

Iago stays in the shadows while he shouts. He has just told Roderigo why: he hates Othello for promoting Cassio over him, but means to keep wearing his uniform. ‘I am not what I am’, from the same scene, is his own summary of the method.

Editions differ on one word. This site’s play text (the Globe edition) prints ‘Is topping your white ewe’. The Folger Shakespeare Library edition prints ‘tupping’, which is the form most people quote. Both words mean the same thing.