Compositional etymology identifying the first element as βάσις "walk, stepping" and the second one as στήριγμα "support". The formal etymon is στηρίζω, στήριγμα, which accounts for the sequence -τηρι-. The original formulation probably was ἀπὸ τοῦ τὴν βάσιν στηρίζειν ἐπ᾽αὐτῆς, τούτεστι ἐρείδειν with the real etymon first and the synonym explaining it in second position. But the compiler dropped the etymon and kept only the synonym ἐρείδειν, which is more usual, therefore more useful for the semantic aspect of the etymology, maybe because στήριγμα was repeated afterwards and he thought that was enough. As a result, form (στηρίζω) and meaning ("to lean") are dissociated and the explanation looks incomplete—and it is. The etymology implies a formal manipulation, change of [s] into [k]. From the semantic point of view, it is a functional etymology as the competing one βάσις + τηρέω.