From earlier -ken, from Middle Dutch -ken, -kin, -kijn, from Old Dutch *-kīn, apparently representing Proto-West Germanic *-ukīn, a double diminutive, from *-uk + *-īn.
The standard Dutch -tje and its variants are derived from -ke(n) through palatalization. Cognate with German -chen, Low German -ken, English -kin, -ock.