the.com/milk of human kindness

The compassion you're born with, before quarterly targets curdle it.

means The natural, instinctive warmth and compassion people have for one another, before circumstance toughens it out of them.

from Coined by Shakespeare in Macbeth (1606) — Lady Macbeth fears her husband is 'too full o' the milk of human kindness' to murder his way to the throne. Milk was the ready image for nourishing gentleness; Shakespeare fused it with 'human kindness' to invent a phrase so apt English simply kept it, long after the play's plot moved on without it.

coined by
Shakespeare, Lady Macbeth doubts her husband has it
original context
she says it like an insult, not compliment
irony
phrase survives, Macbeth's kindness did not
modern use
usually said about someone right before they disappoint you
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