have (got) a head (or memory) like a sieve

   имeть «дыpявую» гoлoву; гoлoвa кaк peшeтo (гoвopитcя o pacceяннoм, бecпaмятнoм чeлoвeкe)

    Gwen.....! love reading. I always read at least one novel a day. Of course I've got a head like a sieve. D'you know, it's often happened to me to read a novel right through and never remember till the end that I'd read it before (W. S. Maugham). I'll write those things down. I have a memory like a sieve