Did you even read the definition you provided here?
Yep and judging from your post you are the one not reading the given definitions.
a : to decrease the flow of (as steam or fuel to an engine) by a valve
b : to regulate and especially to reduce the speed of (as an engine) by such means
c : to vary the thrust of (a rocket engine) during flight
If you actually read what it says it tells you that you can throttle up (generally open the valve) and down (generally closing the valve). If you read b and c there is even more emphasis on the part that it can go up and down. That's what words like "regulate" and "vary" mean!
Without a throttle ALL engines would run at FULL speed/RPM. There would be no restriction to the amount of fuel entering the engine which means your car (for example) would run at maximum RPM at all times.
The way you are defining things means that it can be used only once. You can only turn down the throttle so when the throttle is completely turned down you need to buy a new engine. That's what you are saying here. Obviously you can open and close the valve as one of the definitions says as well as some of the others. You are taking the wording very very literally. This is not how it works with CPUs.
The meaning of throttle is to restrict.
No it's only one of the meanings as dictionaries like Merriam Webster already point out. See below.
Again Turbo boost is the complete opposite of Throttle. I would recommend you take combustion engines 101 before you argue with me again on this. I'm going to assume you have never worked on a combustion engine of any type since you clearly do not understand the definition of Throttle.
Again Turbo Boost is exactly throttling because it varies/controls/regulates the speed of the processor as defined in many dictionaries of the English language. Not only is this basic English, it is also basic computer science. Engines are not computers, they work differently so comparing them is very silly.
In this case the meaning is different from what you are used to with combustion engines. Throttling in computers means to control/vary/regulate, there is no emphasis on limiting. Throttling network traffic however works exactly they way you are used to: it's about limiting bandwidth to make sure other things keep on working properly. With CPUs it is aimed at giving you the power that is needed. The entire idea is that the CPU is being used as efficiently as possible, it is not meant to limit (this is very important to understand). The fact that you can save some power and reduce heat is only an added bonus, there is absolutely no guarantee you'll save power and reduce heat (tasks that require lots of computing power will still make things hot and drain power). Some interesting articles on the matter:
CPU throttling &
network bandwidth throttling.
TL;DR: look up the words "vary", "regulate" and "control" in the dictionary and read into how a CPU controls its speed. A CPU does not work exactly the same as a combustion engine.