10 letters · 14 points in Scrabble
verb make strong or stronger
verb gain strength
verb give a healthy elasticity to
originFrom 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.
Etymology from Wiktionary, CC BY-SA 3.0