Sincerely

google

ref

mid 16th century (also in the sense ‘not falsified, unadulterated’): from Latin sincerus ‘clean, pure’.

wiktionary

ref

sincere +‎  -ly, from Latin sincērus(“pure, clean, sound”). 

etymonline

ref

sincerely (adv.)

1530s, "correctly;" 1550s, "honestly," from sincere + -ly (2). As a subscription to letters, recorded from 1702.