The Schakleton family motto is "fortitudine vincimus." This translates as by means of strenth/courage/fortitude/endurance, we conquer, or, in better english, though endurance, we conquer.
A quick grammatical explanation of this. Fortitudine is the ablative of the noun fortitūdo (strength, force, bravery, courage, endurance, manliness, etc.). The use of the ablative here is ablative of means, which describes how something was done and is translated literally as by means of, thus by means of endurance or with/though endurance. Vincimus is the first person, plural, present, active, indicative form of the verb vinco -- which means to conquer, to be victorious, etc. -- and it would translate as we conquer.
Hope this helps.