oncostEtymologyFrom on- + cost, or from Middle English uncost (“additional expense”), from Middle Dutch oncost (“expenses, overhead”) (Modern Dutch onkost), equivalent to un- + cost. Cognate with German Unkosten (“expenses”).nounAdditional costs; extra expenses.Overhead (expense)The cost of upkeep, making and maintaining shafts, roads, pumping, etc.The indirect expenditure incurred for the purpose of increasing the productive power of organised labour.