Gifford's explanation is beside the point:
Slang: back from enforced seclusion at the end of the world, or back from jail.
The punning origin of the expression, alluding to the fact that Dixon has just helped deliver Mrs Purefoy's baby, can be easily documented:
The little squalling, bawling babes,
That nightly break our rest,
Should be packed off to Baby-lon,
To Lap-land, or to Brest.
Western Temperance Journal (1841), 15 April, p. 54
(The poem, called "Geographical Punning", taken from the British Colonial Magazine, was frequently reprinted for the next decades.)
Chapter VIII […] The penalty of loaf-ing in bed. Cold weather. Nurses. Both suggestive of Lap-land.
Chapter synopsis in James Beresford’s Miseries of human life (1853, New York ed.), p. 13
Childhood's home. - Nowhere is there a greater number of infants reared than in Lap-land.
Punch (1862) vol. 42, Almanack p. 5/3
Joyce's medical friends might easily apply a more obscene reading to the maternal lapland. Eric Partridge's Dictionary of Slang and Unconventional English offers:
The female pudenda: low: from 1840.
(though documentary evidence from the 1840s and up to Ulysses is lacking).
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