Acorns!!!!
in dutch theyre called "Eikels" , from the Eikenboom, and this word also means cock, because of the morphological similarity i guess
and like most names of parts of our sex organs, its also a term of abuse to call someone (male)
just for fun here's the list of such words:
Female : doos, muts, kut
Male: Lul, eikel, penis (Raoul Heertje copyright)