What is the conventional meaning of the Latin maxim “Necessitas non habet legem”?

The maxim conventionally means "Necessity knows no law." "Ignorance of law is an excuse.", "Consent always excludes criminal liability." and "Every harmful act is justified if it benefits its doer." do not translate it. It is a general maxim, not an unrestricted defence; BNS section 19 imposes its own requirements, including absence of criminal intention to cause harm and good faith in avoiding other harm.