Mou : nm Lung of slaughter animals (offal).
Brain (familiar to humans).
Stuff someone's slack: make them believe it, make them believe in nonsense (see below).
Slack on someone: physically assault them, beat them.
(To) rust the slack: have a drink.
Calf slack: Heavy, sagging breast of the woman.
Getting him into the slack: getting into a woman's vagina.
The expression "Bourrer le mou": Telling lies - Seeking to deceive.
The slang expression stuff your head which has exactly the same meaning and whose image is that of the person whose head is filled with nonsense, nonsense and other nonsense which enter it all the more easily as the contents of the skull are soft.
It turns out that this expression, which seems to date from the First World War, is only a copy, mou being a slang word designating the brain (among others), a soft thing if there is one.
We also find the form inflate the slack, so true is it that the one who seeks to fill us with the slack pumps air for us. And from this expression, Celine in 1936 drew it's slack to say "that's a lie".