Not so foolish; nothing that foolish; not that stupid.
Origin
Mid 19th century; earliest use found in William Thackeray (1811–1863), novelist. From French pas si bête from pas, negative particle, specifically use of pas + si so + bête stupid.
Definition of pas si bête in US English:
pas si bête
exclamationˌpɑː siː ˈbɛt
rare
Not so foolish; nothing that foolish; not that stupid.
Origin
Mid 19th century; earliest use found in William Thackeray (1811–1863), novelist. From French pas si bête from pas, negative particle, specifically use of pas + si so + bête stupid.