One would like to undertake something but hasn't the energy or strength to do so. For example, Another set of tennis? The spirit is willing but the flesh is weak. Today often used as a rueful admission of weariness or other physical weakness, this idiom was first recorded in the New Testament (Matthew 26:41), where Jesus tells his disciples: “Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak.” A modern equivalent is I would if I could but I can't.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.