It's a "semi" trailer because it doesn't have its own engine, front axles and wheels, transmission, and so on. Thus, "semi-truck" is a nickname for a truck that carries a semi-trailer.
SEMI meaning: 1 : 54114; 2 : 3 / ˈsɛˌmaɪ/ US : a short, heavy truck that has a long trailer attached to the back and that is used to move goods : semitrailer
When a semi-truck gets a flat, using a plug might seem like the quickest fix — but is it really enough to keep a heavy rig safely on the road?
SEMI is a global industry association that connects and supports companies in the microelectronics design and manufacturing value chain.
The meaning of SEMI is semifinal —often used in plural. How to use semi in a sentence.
Semi- is a numerical prefix meaning "half". The prefix alone is often used as an abbreviation when the rest of the word (the thing which half of is being described) is clear from context.
semi (plural semis) (UK, Ireland, Australia, Canada, informal) A semi-detached house. quotations 2008, Elliott Placks, quoted in Helen Isbister, Morris Bryant, Property, Career FAQs, Australia, page 40, I′m selling two side-by-side semis that are currently under construction, a waterfront apartment and a house in Rose Bay.