“Semi truck” is not half a truck, but a truck designed to carry one half the weight of the cargo it is hauling. A semi trailer is one designed to have half of its load (by weight) carried by the tow vehicle. A standard trailer gets difficult and possibly dangerous to tow if the weight carried by the tow vehicle (hitch weight) strays too far outside the 8%-12% range.
And just to add to the confusion, Dodge popularized something called the “hemi engine”–an engine with a “hemi head”, not half an engine. And “hemi head” refers not to “1/2 an engine head” but to the approximately hemispherical (1/2 sphere) shape of the combustion chambers cast/machined into the engine head.
Canadian here, with 50 years in the workforce. I’ve never once been paid semi-weekly or bimonthly. Here, biweekly is every two weeks semi-monthly is every half month. Obviously, that latter is often spoken of as twice a month, which just adds to the confusion between “bi” and “semi”.
The reality is that these words, like most words (at least in English), mean whatever the speaker wants them to mean and consensus can be hard to reach.
I give you the phrase “table the discussion”. Sometimes it means to formally bring something up for discussion. Other times it means setting the discussion aside for future consideration.
Or, my favourite from my childhood, “fat chance” which means that something is even less likely than if it had a slim chance. Granted, that might be more in the line of idiomatic slang, but it stands as part of at least the era’s Canadian English that did have broad consensus and still does, I think.