Green’s Dictionary of Slang

to stun, i.e. to render silly or ‘insensible’; thus sillified.

1859		Sessions Papers Central Criminal Court 10 May 17: I felt great pain from the blows [...] It half sillied me at the time.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 36/1: His governor did it for the lark of the thing, to see him chuck hisself about – sillyfied like.