1
general::
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. spit and polish Close attention to appearance and order, as in With a little spit and polish this house will sell very quickly. This expression originated in the military, presumably alluding to literally shining up something with the aid of a little saliva. There it Also came to mean "too much attention to appearance, and not enough to more important concerns," as in The commander is so concerned with spit and polish that he overlooks the crew's morale. [Late 1800s]
American Heritage Idioms