English Theasurus that shows all the ways the English word is translated from the original Greek and Hebrew.
English Theasurus from the Hebrew and Greek Strong's Dictionary
thankful 2170 ** eucharistos ** {thankful}.
thankfulness 2169 ** eucharistia ** {thankfulness}, (giving of) thanks(-giving).
unthankful 0884 ** acharistos ** {unthankful}.