How is the meaning of 不得已 formed? I am under the impression that Chinese multi-character words form their meaning from that of the characters that form these words. (Let me know if I am mistaken!)

I know that 不得已 means "to have to".
But consider each character, as described on wiki:


  
  不, "not" 
  得, "particle to connect verb with modifying clause/to have to"
  已, "already/then/to stop"
  


How do these three characters form the meaning of the whole expression? Or is there no rule to this?

Also, my understanding is that 得 (dė) is a particle, yet 得 (děi) means "to have to". That said, would the pronunciation of 得 in this expression be děi?

Thanks in advance!
the usage is classical and shows up at least as far back as mencius: 吾豈好辨哉？吾不得已也。Here the meaning is quite literally "I cannot (不) achieve/obtain (得) an end (已)" to my argumentativeness. In other words, i have no choice but to argue. You might compare it with the much more colloquial 不得不.

By the way be careful about the whole multi-character words thing. Some (not I) would insist that they are mere syllables, the characters accidental byproducts of the spoken language. Better comparison would be with a language composed entirely of words like "counterproductive".