Green’s Dictionary of Slang

mother’s blessing n.

laudanum.

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor IV 245/2: My husband’s bedridden, and can’t do nothink but give the babies a dose of ‘Mother’s Blessing’ (that’s laudanum, sir, or some sich stuff) to sleep ’em when they’s squally.