The key ideas behind the term is that of a free-holder (ie owns land and is not a servant) of a reasonable amount of land - ranking below gentleman in the social scale (but like all terms with a long history its meaning changes over time) - one of the earliest Manx reference I have is 1428 in the Garrison roll - "And also Thomas of Aghton has a man that sits at yeoman's board whereas he ought not to sit.
" which indicates the key idea that yeomen are above servants in the pecking order - in the introductory part of the IoM Statutes dating from c.1429 is "Alsoe that noe Yeoman have Meate or Drink except he have been on the Lord his Business but at the Bell." i.e. below any gentlemen who would naturally have been fed at the Lord's expense.