Comes from 《
庄子·
人间世》 (Zhuangzi, 'In the Human World'): "
夫两喜必多溢美之言，
两怒必多溢恶之言" — 'when two parties are both pleased, their words inevitably overflow into excessive praise; when both are angry, into excessive blame.'
溢 ('to overflow') is the key image: praise that has spilled past truth.
