For some reason the name of this traditional filling for pies or accompaniment (ground beef) for totties has been borrowed by the dialect for a variety of reasons.
It can mean nonsense or deliberate untruthfulness: “That’s pure mince you’re talking.” “Do they expect us to believe this mince?” Another meaning is anything nasty or dirty: “What’s this mince on the sleeve of ma coat?”
It is taken as a measure of density, whether of the brain or another substance: “The guy’s as thick as mince.” Some people refer to a pint of Guinness as a pint of mince. A person who seems very quiet or downcast, may be told they are sitting there like a pun (pound) of mince.”
To sicken someone’s mince is to spoil something for him or deflate him: “It didnae half sicken his mince when he didnae get that bonus.”