Two things strike me in reading the OED entry for bottom, n. 1b.
Firstly, the definition itself:
The sitting part of a man, the posteriors, the seat. (Colloq.) Also, the ‘seat’ of a chair.
I’ve never been an advocate for the wholesale replacement of the generic ‘man’ with other words, but in this instance I believe there’s a case for it, if only for the ambiguity. To someone unfamiliar with the language this could be taken to mean that the sitting part of a woman was called something else. I thought the editors had completely revised the earlier letters of the alphabet; if so, I’m surprised they didn’t pick up on this.
Secondly I was struck by the date of the first citation. No earlier evidence for the sense? That’s surprising.
1794-6 E. DARWIN Zoon. (1801) III. 253 So as to have his head and shoulders much lower than his bottom
It doesn’t push it much further back and it’s rather an implied than actual cite but there is the hilarious passage in Boswell’s Life of Johnson, published in 1791 but reporting a conversation of a decade earlier.
Talking of a very respectable authour, he told us a curious circumstance in his life, which was, that he had married a printer’s devil. REYNOLDS. ‘A printer’s devil, Sir! Why, I thought a printer’s devil was a creature with a black face and in rags.’ JOHNSON. ‘Yes, Sir. But I suppose, he had her face washed, and put clean clothes on her. (Then looking very serious, and very earnest.) And she did not disgrace him; the woman had a bottom of good sense.’ The word bottom thus introduced, was so ludicrous when contrasted with his gravity, that most of us could not forbear tittering and laughing; though I recollect that the Bishop of Killaloe kept his countenance with perfect steadiness, while Miss Hannah More slyly hid her face behind a lady’s back who sat on the same settee with her. His pride could not bear that any expression of his should excite ridicule, when he did not intend it; he therefore resolved to assume and exercise despotick power, glanced sternly around, and called out in a strong tone, ‘Where’s the merriment?’ Then collecting himself, and looking aweful, to make us feel how he could impose restraint, and as it were searching his mind for a still more ludicrous word, he slowly pronounced, ‘I say the WOMAN was FUNDAMENTALLY sensible;’ as if he had said, hear this now, and laugh if you dare. We all sat composed as at a funeral.
The scene is priceless!