παρά-τριψις · para-tripsis — LSJ
rubbing against one another, friction, νεφῶν Epicur. Ep. 2p.45U. (pl.), cf. Arist. Mu. 395b5, Thphr. Lap. 45, Ph. Bel. 78.10, D.S. 3.36, Placit. 3.3.11 (pl.), M.Ant. 6.13 ; τῇ τοῦ ποδὸς πρὸς τὴν γῆν π. Chrysipp. ap. S.E. P. 1.70.