The spirit is willing but the flesh is weak

What's the meaning of the phrase 'The spirit is willing but the flesh is weak'?

A statement of the difficulty in living up to the high moral standards that one has set oneself.

What's the origin of the phrase 'The spirit is willing but the flesh is weak'?

From the Bible, Matthew 26:41 (King James Version):

Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak.