I thought that the word please came from the plural of plea. But then why is it please instead of just pleas? Why the e? Are "plea" and "please" really related to each other?
Surprisingly, plea and please are not derived from each other. The verb please comes from Old French, and ultimately from Latin:
The word plea also comes from Old French and ultimately from the same Latin word as please, but the stems were already different in O.Fr., and there is not and never has been any morphological relationship to please in English.
All etymologies from Online Etymological Dictionary.