The sound a clumsy action would make. For example if a partner were walking into BodyCare and bumped their handbag into the Fructis display the sound would be 'Donk!'. The sound may not actually occur in real life and in this instance you yourself being eductaed in the word should provide it.
Pretty girl falls to knees descending stairs of Brighton Nightclub...you would say..."DONK!!"
"DONK!, you totally knocked that brolly over in the porch"