Usually when two characters combine I can puzzle through how they obtained their combined meaning.
However, even after years of study I have no idea why 即使 means even if.
I’m looking to better understand its etymology.
即使 ("assuming that", "even if") introduces a concessive conditional clause. This type of clause refers to an event that may not necessarily be the case, but that you are assuming to be the case in order to argue that it would not prevent another event (in the main clause) from happening.
The logic behind 即使 is that when you are assuming that first hypothetical event, what you are doing is "causing" it to happen, if only for the sake of the argument.
That's where 使 comes in. 使 means "to cause/send/tell someone to do something" and is often used in causative constructions. When you are "causing" an event as a thought experiment (as you do when you use a concessive conditional), 使 is there to indicate that you're forcing the event to happen, like make-believe, i.e. that you're assuming it.
This sense of 使 is literally spelled out in 假使 ("pretend-causing" => "in the event that"), a synonym of 即使. While 假使 is unambiguously hypothetical, 即使 is more neutral, with 即 only standing in for the linking word "就是".
It takes work to answer the question. We Chinese usually use the expression quite smoothly and never think of it. To reply to your question, I've checked the dictionary but pity that there are no explicit explanations for how this 2-word was initially comprised. Even so, I'll try to explain it according to my comprehension. We can detach
使, which further can be understood as abbreviations of
假使. More confused, right? In fact,
即使 = 即便 + 假使
即使 acts as a conjunction, followed by an adverbial clause of concession, meaning negation. With this in mind, then we go back to see
即便 + 假使
便， both means right away, at once.
on the other hand,
假使 means suppose(
假) somebody makes something happen(
So the combined meaning is even immediately makes something happen (hypothesis) , it's unavailing.