Although the word 'freight' can have several different meanings, where it comes from, or what language experts would call its entymology, is fairly straightforward.
Freight can mean the actual goods transported by train, lorry, aeroplane or ship or, more generally, the transport of bulk goods by these methods. The word can also be used to mean the final cost or bill of the transportation of such goods. A lesser used meaning is for something to be 'freighted' with meaning, in that something is meaning heavily implied by what is said, but this meaning of the word 'freight' is falling into disuse.
Where the Meaning of 'Freight' is Derived From
The word is believed to have originated in late Middle English (an era roughly spanning the three centuries between the late 12th and the late 15th century), and meant 'hire of a ship for transporting goods.' The English word came about from the contemporary German/Dutch word vrecht, or vracht which meant 'ship's cargo.' This is similar to the origins of the word 'fraught,' as transporting large cargoes vast distances across the sea in those times was a hazardous business - transporting freight was fraught with danger! To learn more about the origins of the English language, watch this interesting BBC documentary: