"You are not your f***ing khakis" - What does "khakis" exactly mean in the Fight Club movie?

I'm surprised no one has mentioned what khakis actually means in some non-rhotic accents. Non-rhotic accents are ones in which the r sound is only pronounced when it's followed by a vowel (e.g. right); the r is silent elsewhere. Khakis is a humorous pronunciation-spelling of car keys in some areas of the USA (such as Boston). The r is silent because it's followed by a consonant, so it's pronounced KAH-KEEZ which could alternatively be spelt khakis.

Here's an illustration from A Historical Phonology of English by Donka Minkova (p274):

I assume it also means car keys in the quote in the OP:

You are not the car you drive
You are not your fucking khakis! [car keys]

"Khakis" is often used to refer to pants (that are khaki-colored, or made of the khaki textile). For example, Old Navy has a khakis page that lists various pants and one specific to men's khakis.

In the context of the movie, khakis are another 'material good' that Tyler denounces, like cars, ikea furniture, etc.