Meaning of the word "nothing comes out of the sack but what was in it" in English

What does "nothing comes out of the sack but what was in it" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

nothing comes out of the sack but what was in it

US /ˈnʌθ.ɪŋ kʌmz aʊt əv ðə sæk bʌt wʌt wəz ɪn ɪt/
UK /ˈnʌθ.ɪŋ kʌmz aʊt əv ðə sæk bʌt wɒt wəz ɪn ɪt/
"nothing comes out of the sack but what was in it" picture

Idiom

you cannot get more out of something than what was originally put into it; a person's actions or words reflect their true character or knowledge

Example:
He tried to sound like an expert, but nothing comes out of the sack but what was in it.
If you don't study, you can't expect to pass; nothing comes out of the sack but what was in it.