Han’gul Korean hapax legomenon [Grk hápax legómenon ‘said once’]

Linguistic expression with only one attested occurrence and whose meaning is often, therefore, difficult to ascertain. Hapax legomena serve as a basis for defining the morphological notion of productivity in Baayer and Lieber (1991). ( also pseudomorpheme, semimorpheme)

Reference

Baayer, H. and R.Lieber. 1991. Productivity and English derivation. Linguistics 29. 801-3.