Strengthen

wiktionary

ref

From rare Middle English strengthenen (14th c.), from earlier strengthen (12th c.), where -en is the infinitive ending. Probably the original form was reinterpreted as strength +‎ -en around the time when the infinitive ending was being apocopated in late Middle English.

etymonline

ref

strengthen (v.)

late 14c., from strength + -en (1). Related: Strengthened; strengthening; strengthener. Earlier verb was simply strength (12c.).