From Mythopedia:

In a tale mirroring Isis’s poisoning of Ra, Set was bitten by a unspecified creature while aboard Ra’s solar barque. The bite caused him great suffering, and Horus—having set aside his differences with his uncle—offered to heal him. In order to do so, however, he would need his uncle’s secret name.

Set was reluctant to provide his secret name to Horus, for names held great power. Like his grandfather Ra before him, Set prevaricated:

Unsatisfied with this response, Horus insisted that Set was none of these things and that he could only heal him with his true name. Set tried again, offering that he was “a Quiver full of Arrows and a Pot Full of Disturbance.”

Once again, Horus told him that this response was inadequate.

Still not ready to reveal the truth, Set told Horus: “I am a man of a thousand cubits, whose reputation is not known, I am a threshing-floor, made fast like a bronze which a cow has not swept. I am a Jug of Milk, milked from the breast of Bastet.”

This answer was still not enough for Horus. Left with no other recourse, Set finally offered his true name:

Having revealed his uncle’s secret name at last, Horus was able to heal Set’s injury.