In Kassapa Buddha's time she had been a nun well established in the precepts. But she reviled an arahant Therī by calling her a prostitute, and for this she was born in purgatory. In the present age she was the daughter of a rich and distinguished citizen of Benares but, because of her former evil speech, became a prostitute in Rājagaha. Having heard the Buddha preach, she entered the Order of the bhikkhunis. Wishing to obtain the higher ordination from the Buddha, she set out for Sāvatthi, but was waylaid and stopped by libertines. So she sent a man to ask the Buddha's advice and he permitted her to be ordained by a messenger (Thig.vv.25-6; ThigA.30ff.; Vin.ii.277; Ap. ii. 610-11). Her case established a precedent (Sp.i.242). Later she attained arahantship.
It has been suggested (VT.iii.360, n.3; and VT.ii.195-6, n.3) that her name "half Kāsī" might mean that she charged five hundred pieces from her patrons. For, according to Buddhaghosa, Kāsī means one thousand, and anything worth one thousand is called kāsiya.
Another explanation is, however, given by Dhammapāla (ThigA.32). The revenue which accrued to the king for one day from Kāsī was a thousand. Addhakāsī's patrons had to give a like sum to spend a night with her. This is referred to in one of the verses attributed to her in the Theragāthā (v.25). For this reason she was called Kāsī. But later, many men, not being able to afford a thousand, would pay half the amount and spend the day with her. As a result she became known as Addhakāsī.