A verb in The Etymologies of the 1930s appearing in its (Noldorin) infinitive form N. thribi “to scratch” derived from the root ᴹ√SRIP of the same meaning (Ety/SRIP).

Neo-Sindarin: I would adapt this verb as ᴺS. rhib- “to scratch” for purposes of Neo-Sindarin, as suggested in Hiswelókë’s Sindarin Dictionary (HSD), since primitive sr became voiceless rh in Sindarin of the 1950s and 60s.