Ultra posse nemo obligatur is a Latin legal term, meaning, "No one is obligated beyond what he is able to do."
Ultra posse nemo obligatur has its origin in the Roman law. The expression can be found in Justinian's Digesto.
A common variant of the phrase ultra posse nemo obligatur is ad impossibilia nemo tenetur.
References
Ultra posse nemo obligatur Wikipedia(Text) CC BY-SA