Man knocking on the front door of an old, derelict house out in the countryside. Knocking is also a phrase in England that means getting older hence “the old man who was ‘knocking on’ in age, knocked on the door of the derelict house”. Perhaps knocking means that the person is either becoming elderly, or even approaching death. There are so many strange phrases in the English language. ‘Knocking on’ can be changed to ‘getting on’ or even ‘clocking on’ which can also mean getting your clock card punched by a machine at the start of a factory shift. The more you learn the more there is to find out!
Mallaband-Brown 