He would fain stay.
An example of fain is a woman who is eager to marry her fiance.
Origin of fain
- Middle English from Old English fægen joyful, glad
From American Heritage Dictionary of the English Language, 5th Edition
- From Old English fægen, akin to Old Norse feginn (“glad, joyful”), Gothic (faginon, “to rejoice”), Old Norse fagna (“to rejoice”). Compare Gothic (*fahs, “glad”).