1. Mr. is a shortened form of mister. You use Mr. before a man's name, usually the last name or the full name. [Simple Wikepedia]
2. Mr. [Wikipedia]
Mister, usually written in its abbreviated form Mr. (US) or Mr (UK), is a commonly used English honorific for men under the rank of knighthood. The title 'Mr' derived from earlier forms of master,....
3. History and Etymology for mR [Webster]
Noun
Mr. from Middle English, abbreviation of maister master;
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I searched for the explanation about 'Mr.'
Q1 Is Mr. a shortened form of 'Mister' or 'maister' or 'master'?
Q2 In #2, is 'maister' a word in Middle English and does 'maister' mean 'master'?
https://www.jiskha.com/questions/1821488/is-mr-a-contraction-of-mister-or-master
https://www.etymonline.com/search?q=mister
(the first two)