Definition of Sundry
Sundry can mean various, miscellaneous, or diverse.
Definition of Sundry Debtors
Sundry debtors could be referring to a company's customers who rarely make purchases on credit and the amounts are not significant.
Example of Sundry Debtors
I suspect that the term sundry was more common when bookkeeping was done manually. For instance, prior to the low cost of computers and accounting software, the bookkeeper had to add a page to the company's subsidiary ledger book for every new customer. Adding a new page for every occasional customer could result in a subsidiary ledger book that was unwieldy. Therefore, it was more practical to have one page entitled sundry on which those occasional customers' small transactions were entered.
With the efficiency and low cost of today's accounting systems, the need for classifying customers and accounts as sundry has been eliminated.