When I got to the attic, things were pretty confused. Grandfather had evidently jumped to the conclusion that the police were deserters from Meade’s army, trying to hide away in his attic. He bounded out of bed wearing a long flannel nightgown over long woolen pants, a nightcap, and a leather jacket around his chest. The cops must have realized at once that the indignant white-haired old man belonged to the house, but they had no chance to say so. ‘Back, ye cowardly dog!’ roared grandfather. ‘Back t’ the lines ye goodaam Lily-livered cattle!’ With that, he fetched the officer who found the zither a flat – handed smack alongside his head that sent him sprawling. The others beat a retreat, but not enough; grandfather grabbed zither’s gun from its holster and let fly. The report seemed to crack the rafters; smoke filled the attic. A cop cursed and shot his hand to his shoulder. Somehow, we all finally got downstairs again and locked the door against the old gentleman. He fired once or twice more in the darkness and then went back to bed. ‘That was grandfather’, I explained to Joe, out of breath. ‘He thinks you’re deserter.’ ‘I’ll say he does,’ said Joe. 

Explanation:

 
Things were a little chaotic when the narrator arrived to the attic. Grandfather had apparently assumed that the police officers were Meade's army deserters seeking to hide in his attic. He leapt from his bed, dressed in a long flannel nightgown, long woollen pants, a nightcap, and a leather jacket across his chest. The cops must have immediately realised that the enraged white-haired old man belonged to the house, but they didn't have the opportunity to say so. Grandfather hurled curses at the police. He said, 'Back, you cowardly dog!' and 'You goddamn Lily-livered cattle, get back to the lines!'
 
With that, he got hold of the police officer who had found the zither and hit him across the head that knocked him out. The others tried to flee, but they weren't fast enough; grandfather took zither's revolver from its holster and fired. The gun shot shattered the rafters, and smoke flooded the attic. A cop cursed and held his shoulder with his fist. The group eventually made their way downstairs and locked the door behind the grandfather. The grandfather fired a few more shots in the dark before retiring to his bed.
 
When the group was downstairs, the narrator explained to Joe that the old man was his grandfather. He also explains how the grandfather thinks that the police were deserters from the Maede's army.  Joe responds by stating ‘I’ll say he does', suggesting a tone of sarcasm.
  
Meanings of difficult words:
 
Sl. No
Words
Meanings
1
Flannel(Of a quality) which you are born with, or which is present naturally
2
Ye(A colloquial slang) You
3
Cowardly; Lily-liveredLacking courage
4
T'(A colloquial slang) To
5
Fetch Go for and then bring back (someone or something) for someone
6
Smack A sharp slap or blow, typically one given with the palm of the hand
7
SprawlTo sit, lie, or fall with one's arms and legs spread out in an ungainly way
8
BeatTo move with sudden speed 
9
RetreatAn act of moving back or withdrawing
10
Holster A holder for carrying a handgun or other firearm, typically made of leather and worn on a belt or under the arm
11
RafterA beam forming part of the internal framework of a roof
12
ReportA sudden loud noise of or like an explosion or gunfire
Reference:
State Council of Educational Research and Training (2019). English Standard-10. The Night the Ghost Got In - James Grover Thurber (pg. 30 - 33). Published by the Tamil Nadu Textbook and Educational Services Corporation.