Her husband, Caecina Paetus, was ordered by the emperor Claudius to commit suicide for his part in a rebellion but was not capable of forcing himself to do so.
Arria wrenched the dagger from him and stabbed herself, then returned it to her husband, telling him that it didn't hurt ("Paete, non dolet!").
Her story was recorded in the letters of Pliny the Younger, who obtained his information from Arria's granddaughter, Fannia.
Pliny records that Arria's son died at the same time as Caecina Paetus was quite ill. She apparently arranged and planned the child's funeral without her husband even knowing of his death.
If emotion threatened to get the better of her, she excused herself from the room and would, in Pliny's words, "give herself to sorrow", and then return to her husband with a calm demeanor.