The term (with the sense of ‘measuring rod’) is otherwise not attested (according to the OED, the earliest English usage – ‘an apparatus for sounding the sea’ – dates only from 1811), but must be related to the noun sonde (DMF: ‘Instrument dont on se sert pour connaître la profondeur de l'eau’) and the verb sonder (DMF: ‘Mesurer au moyen de la sonde la profondeur de l'eau’). Neither the noun or the verb are attested in Anglo-Norman (and their equivalents appear in English from the last quarter of the fifteenth century), leaving this lexeme, using an -our agent-suffix, an isolated instance.