It is a musical device associated with the lament topic. It consists of a melodic motive built on a descending minor second, imitating the sob of someone weeping.
«[The pianto], the motive of a falling minor second, has represented a lament since the sixteenth century. At first it always accompanied the textual idea of weeping – words like “pianto” or “lagrime” – but it soon began to signify merely grief, pain, regret, loss – in other words, the indexicality of its immediate object. During the eighteenth century the related idea of the sigh replaced that of weeping. For this reason Riemann, finding this figure in early Classical music, called it “the Mannheim sigh”».
Referencias: Monelle, 2000: 17; Grimalt, 2020: 33-35.