Readers today may not realise that Buck Mulligan punishes the vegetation using the language of a dog owner of the time. "Sir" was a common term of address to a dog:
Down, Romulus, down, sir," said he to the dog, who had jumped up at the kids. "How dare you, sir? Down, I say.
Frederick Marryat, Masterman Ready (1836), ch. 30
Gently press the dog down, repeating "Down! Down Sir!" hold over him a twig or a whip; if he resist, (as most probably he may,) use the whip very lightly, and increase in severity, according to the obstinacy of the animal [...]
Francis Butler, Breeding, training, management, diseases, &c. of dogs (1860), p. 73
Let me hold him! He will obey me!" she cried, placing her little hand on the great dog's neck. "Down, Emperor! Down! How dare you! Down sir!
Marie Corelli, Sorrows of Satan (1895), ch. 20
In "The Dead" we find the same term of address applied to a horse, as 'the late lamented Patrick Morkan' reached the 'statue of King Billy' on his way to a military review before his mill-horse Johnny reverted to type and started circling King William:
"Round and round he went," said Gabriel, "and the old gentleman, who was a very pompous old gentleman, was highly indignant. 'Go on, sir! What do you mean, sir? Johnny! Johnny! Most extraordinary conduct! Can't understand the horse!"
Perhaps the most nicely observed disambiguation of ‘Sir!’ comes from the German traveller in Britain, Charles Moritz (Karl Philipp Moritz), in a letter of 15 July 1782:
The word Sir! in English has a great variety of significations. With the appellation of Sir, an Englishman addresses his King, his friend, his foe, his servant, and his dog […] Sir! in a surly tone, [signifies] a box on the ear at your service! To a dog it means a good beating.
Travels (ed. 2, 1797), p. 260
For Stephen, who has memories of a traumatic confrontation with Mulligan's dog, this little scene may contribute to his ever deepening resentment against the ‘usurper’ of the tower.
The dog’s bark ran towards him, stopped, ran back. Dog of my enemy. I just simply stood pale, silent, bayed about. Terribilia meditans. A primrose doublet, fortune’s knave, smiled on my fear.
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