faux naif
Concise Oxford English Dictionary © 2008 Oxford University Press:
faux naif /ˌfəʊ nʌɪˈiːf/
▶adjective artificially or affectedly naive. ▶noun a person who affects naivety.
– origin from Fr. faux ‘false’ + naïf ‘naive’.
'faux naif' also found in these Oxford entries:

