non est factum
Concise Oxford English Dictionary © 2008 Oxford University Press:
non est factum /ˌnɒn ɛst ˈfaktəm/
▶noun Law a plea that a written agreement is invalid because the defendant was mistaken about its character when signing it.
– origin L., lit. ‘it was not done’.
'non est factum' also found in these Oxford entries:

