but (adv., prep.) [<--] Old English butan, buton "unless, except;
from West Germanic * be-utan, a compound of * be- "by" (see by)
+ * utana "out, outside; from without," from ut "out" (see out (adv.)).
Not used as a conjunction in Old English. As a noun from late 14c.
How did the two bolded West Germanic etymons (* be- + ut), combine to mean unless?
My guess: If an object is out by you, then the object is
outside (of) you. To wit, you are
without the object.
But how does this evolve to mean unless?
Please expose, explain, and bridge all hidden, missing semantic drifts and links. What is a right way of interpreting the etymology, to understand how the semantic jumps abstracted and strayed from the original literal meaning?
I don't quote OED, whose etymology stops at Old English and so doesn't retrograde enough.