actually early word was Manske - see http://www.ee.surrey.ac.uk/Contrib/manx/iomnhas/v023p249.htm
also found in 1428 Garrison roll http://www.ee.surrey.ac.uk/Contrib/manx/history/peelc/pt7.htm

Mann prob is an abbreviation of latin Manniae (of/relating to) Man(n) - Maniiae was not possible as this meant of madness! thus a double nn was adopted - W W Gill also had a go at name - see http://www.ee.surrey.ac.uk/Contrib/manx/history/name_man.htm