Pope and Lady Mary had once been friends. Apparently Lady Mary rejected Pope's romantic advances, and after that their friendship deteriorated. Pope often attacked her as Sappho; she replied in 1733 with this poem. It may have been cowritten with Lord Hervey, whom Pope attacks as Sporus.
In two large Columns, on thy motly Page,
Thine is just such an Image of his Pen,
Horace can laugh, is delicate, is clear,
If He has Thorns, they all on Roses grow;
Satire shoud, like a polish'd Razor keen, 
But if thou see'st a great and gen'rous Heart,
Not even Youth and Beauty can controul
When God created Thee, one would believe,
Nor think thy Weakness shall be thy Defence; 
If none with Vengeance yet thy Crimes pursue,
Is this the Thing to keep Mankind in awe,
If none do yet return th'intended Blow;
Nor thou the Justice of the World disown,
1. Pope's Horatian imitations were published with Horace's Latin on the left-hand page (the verso) of every opening, and his imitations on the right (the recto).
2. Motley, "mingled" or "confused." Since jesters traditionally wore motley-colored clothes, the word is often associated with them.
3. Stripe, "A weal; or discolouration made by a lash or blow" (Johnson).
4. That is, where obscenity pretends to be satire.
5. Pope had translated Homer's Iliad and parts of his Odyssey. Although the translations were very popular, scholars accused him of having an inadequate understanding of Greek.
6. In other words, your version of Homer is as close to him as you are to a human being. Pope was very short (four foot six) and hunchbacked.
7. Burlesque, "Tending to raise laughter, by unnatural or unsuitable language or images" (Johnson).
8. Rail, "To use insolent and reproachful language; to speak to, or to mention in opprobrious terms" (Johnson).
9. Numbers, "Verses; poetry" (Johnson).
10. Lady Mary was noble; Pope came from less distinguished parents.
11. Stew, "A brothel; a house of prostitution" (Johnson).
12. Spleen, "Anger; spite; ill-humour" (Johnson).
13. Dart, "arrow" or "spear."
14. Sequel, "whatever comes after."
15. An allusion to God's punishment of the serpent and Eve after the fall in Eden, recounted in Genesis 3:15: "And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel."
16. Wants, "lacks."
17. The fretful porpentine is a quotation of Hamlet.
18. Pope alludes to this line in his Epistle to Arbuthnot 213 ("Who but must laugh, if such a man there be?").
19. The first bold assassin is Cain, who killed his brother Abel.