iliacus Syllabification: (i·li·a·cus)Pronunciation: /iˈlīəkəs/ (also iliacus muscle)Definition of iliacus noun Anatomy a triangular muscle that passes from the pelvis through the groin on either side and, together with the psoas, flexes the hip. Origin: early 17th century: from late Latiniliacus in other Oxford dictionaries Definition of iliacus in the British & World English dictionary