Entry 1 sense Webster, 1913 Assibilation · As·sib·i·la·tion · IPA /əˌsɪbɪˈleɪʃən/ 01 n. Change of a non-sibilant letter to a sibilant, as of -tion to -shun, duke to ditch. 1. Change of a non-sibilant letter to a sibilant, as of -tion to -shun, duke to ditch.