Curse used to describe no-name, generic, cheap brands of products that are a pale imitation of the real thing and frequently disappoint the consumer.
Can be used to describe Petrol Companies, Phone companies, Utilities companies and other service providers who overcharge for essential commodities that you can't live without (usually when their name also sounds like Shitex).
Occasionally used as a singular curse when one is vexed.
Derived from the Scottish word shite
and the commonly used suffix tex
which is presumably Texan in origin.