LSJ
bend in, ἐν δὲ γόνυ γνάμψεν, i.e. caught the back of the knee with his foot so as to trip him up and throw him, Il. 23.731.
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)
Pape
einbeugen, in tmesi ἐν δὲ γόνυ γνάμψεν Il. 23.731.
Pape, Griechisch-deutsches Handwörterbuch (3. Aufl., 1914)