edgewise

also edge-ways, "with the edge turned forward or toward a particular point," 1560s, from edge (n.) + way (n.). First attested form of the word is edgewaie; the adverbial genitive -s appears by 1640s. Edgewise (1715) appears to be a variant, based on otherwise, etc. See edge (v.).

As if it were possible for any of us to slide in a word edgewise! [Mary Mitford, "Our Village," 1824].

To edge in a word in this sense is from 1680s.