This is the 3903rd most frequent Chinese word.
不已
"Endlessly" or "unceasingly."
Here, '不已' emphasizes the continuous and unceasing nature of the regret.
他对这件事感到后悔不已。
He felt endless regret about this matter.
In this case, '不已' describes the ongoing and perpetual state of worry induced by the quarrel.
他们的争吵让人担心不已。
Their quarrel caused continuous worry.
'不已' is utilized here to convey the unending and persistent emotion of happiness experienced by the little girl.