Definition from Wiktionary, the free dictionary
From 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 Unkost (“expenses”).
oncost (plural oncosts)
- Additional costs; extra expenses.
- (finance) Overhead (expense)
- (coal-mining) The cost of upkeep, making and maintaining shafts, roads, pumping, etc.
- (Britain dialectal, Scotland) The indirect expenditure incurred for the purpose of increasing the productive power of organised labour.