The truss rod was invented by a guy named Rod. As it appeared to be a very reliable means of keeping the neck of a bass in shape, people called it the 'trust rod device'.
I thought it was because Rod was also a part time model for medical catalogues and was reknown for modeling hernia trusses. He was forced into this line of work because he couldn't make a living just selling guitar parts so he was known as Truss Rod to his friends and family.