Fain \FEYN\ , adverb;
1. Gladly; willingly
1. Content; willing
2. (archaic) Constrained; obliged
3. (archaic) Desirous; eager
This word comes from Old English fægen or fagen, meaning "glad, cheerful, happy, joyful, rejoicing. There are similar words in Old Saxon (fagan), Old Norse (feginn, "glad"), Old High German (faginon), and Gothic (faginon, "to rejoice"), all of which derive from a common Germanic root. That Germanic root possibly comes from Proto-Indo-European *pek- ("to make pretty"). The adverbial usage dates to around 1200.
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Wednesday, January 19
Etymologies come from the Oxford English Dictionary and/or Etymonline.com