Did your mother speak Xhosa? If so, I imagine she chose to drop the w and use one of the English pronunciations of th in order to make it easier for non-Xhosa speakers.
In Xhosa it is, more or less, taan
DEE weh. Long a, long e, short e. And the th- never has an English th or dh sound - the h indicates that the t is unvoiced.