The definition of this word is hard to understand given the context in which you mention it. The word refers to a scientific way of thinking that is based on evidence rather than on simply believing what others tell you. So you might say that someone with an "empirical habit of thought" (as the book says) is someone who likes to experiment and find the truth for themselves based on the evidence they find.
You can see where this would not happen much in what the book calls a "strictly regimented society." In a society like the one where Winston lives, people are not supposed to think for themselves and find evidence for themselves. Instead, they are simply supposed to believe what the Party tells them, no matter if there is evidence to support it or not.
So, the word empirical in this context refers to a way of thinking that demands proof and evidence before it will believe in something.