the spirit is willing, but the flesh is weak

Meanings

1. As much as one wishes to achieve something, the frailties of the human body often make it impossible. Sometimes extended to jocular or organizational referents.

Etymology

From the Bible's account of Jesus' last night in Gethsemane. Matthew 26:41: Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak.