The story
Possibly an elaboration of more power to you, with the speaker expressing the wish that the person addressed will have more strength to achieve their goal.
Etymology from Wiktionary, CC BY-SA 4.0.
more power to your elbow
more pow·er to your el·bowToday it means: Used to express admiration of a person coupled with wishes for their continued success in an endeavour of their own choosing: all the best; good luck. Full definition →