The name Thrale or Thrall is thought to derive from the Old English term 'præl' or the Old Norse term 'præll', meaning …
a villein, serf, bondman.
It is likely to have arisen from the requirement to work three or four days a week for the Lord of the Manor. 1
Links
[1] http://thrale.com/user/login?destination=comment/reply/310%23comment-form
[2] http://thrale.com/user/register?destination=comment/reply/310%23comment-form
[3] http://books.google.co.uk/books?id=fPoujUPs3hYC&pg=RA1-PA445&vq=thrale&dq=The+Origins+of+English+Surname.+P.H.+Reaney.&source=gbs_search_s