Green’s Dictionary of Slang

jack in the (low) cellar n.

an unborn child.

1751		Smollett Peregrine Pickle (1964) 48: When his companions drank to the Hans en kelder, or, Jack in the low cellar, he could not help displaying an extraordinary complacence of countenance.
1785		Grose Classical Dict. of the Vulgar Tongue n.p.: Hans in kelder, jack in the cellar, i.e. the child in the womb; a health frequently drank to breeding women, or their husbands.
1892		Aitken in Satires of Andrew Marvell II 16: [Footnote] For Hans-in-Kelder. 66. – Jack-in-the-cellar.