The story
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.
Etymology from Wiktionary, CC BY-SA 4.0.
the spirit is willing, but the flesh is weak
Today it means: 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. Full definition →