Dutch and Low German pet name for Geerta, from Old German "spear maiden" or "spear strength".  Geerta is a feminine form of Geert, itself a variant of Gerhard and Gerard.

Geertje is also a Dutch pet name for Geertruida/Gertruida, variants of Gertrude.

In Dutch, Geertje may also be a given name in its own right [see FAQ 16].