means the genus case, the case which shows the genus; thus, a bird of the air, of the sea, of the marshes, etc. The part in italics shows to what genus the bird belongs. Our's is the adjective sign, the same as the Sanskrit syâ, as udaka (water), udakasya (of water, or aquatic). So in Greek, demos (people), demo-sios (belonging to the people), or genitive demosio, softened into demo-'io. In Chaucer, etc., the genitive is written in full, as The Clerkes Tale, The Cokes Tale, The Knightes Tale, The Milleres Tale, etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894