Archaic. constrained; obliged: He was fain to obey his Lord.
Archaic. glad; pleased.
Archaic. desirous; eager.
(usually with would) archaic willingly; gladlyshe would fain be dead
willing or eager
Old English fægen, fagen “glad, cheerful, happy, joyful, rejoicing,” from a common Germanic root (cf. Old Saxon fagan, Old Norse feginn “glad,” Old High German faginon, Gothic faginon “to rejoice”), perhaps from PIE *pek- “to make pretty.” As an adverb, from c.1200.