DeSay and Say/de Say Surnames

Say
English (of Norman origin): habitational name from Sai in Orne or Say in Indre, perhaps so called from a Gaulish personal name Saius + the Latin locative suffix -acum.
English: metonymic occupational name for a maker or seller of say, a kind of finely textured cloth, Middle English say (from Old French saie, Latin saga, plural of sagum ‘military cloak’). In some instances the surname may have arisen from a nickname for an habitual wearer of clothes made of this material.
Southern French: topographic name from saix ‘rock’ (Latin saxum), or a habitational name from a place named with this word, for example, Say in Loire, Saix in Tarn and Vienne, Le Saix in Hautes-Alpes, or Les Saix in Isère.

Dictionary of American Family Names, Oxford University Press, ISBN 0-19-508137-4