He was a fattish but active man of paralysing stupidity, a mass of imbecile enthusiasms — one of those completely unquestioning, devoted drudges on whom, more even than on the Thought Police, the stability of the Party depended.

Can someone translate this for me? Especially the parts "paralysing stupidity, a mass of

Question

He was a fattish but active man of paralysing stupidity, a mass of imbecile enthusiasms — one of those completely unquestioning, devoted drudges on whom, more even than on the Thought Police, the stability of the Party depended.

Can someone translate this for me? Especially the parts "paralysing stupidity, a mass of

Answer

Paralysing stupidity means you are so stupid it makes you not able to function.

A mass of imbecile enthusiasms 
Imbecile = stupid
Enthusiasms = excitements

He was excited about idiotic or stupid things

Total meaning:

He was so stupid he never questioned anything and just worked reliably which is why he was stable and the party could depend on him. He wouldn't do anything spontaneous.

Answer

'Paralysing' means someone cannot move. 'Stupidity' means someone is not clever. A 'mass' is a large amount of something.

I hope that helps :)