The sense of this word has suffered weakening since the time of the King James Version. Then it implied utmost exertion and success; now rather forlorn hope and possible failure. Thus the Revised Version (British and American) reads "giving diligence," "give diligence," for the King James Version "endeavoring," "endeavor," in Eph 4:3; 2Pe 1:15, respectively; but "endeavored" is suffered to remain in 1Th 2:17 (spoudazo, "hasten," "exert oneself"). Compare also Ac 16:10, the King James Version "endeavored," the Revised Version (British and American) "sought" (Greek zeteo, "seek").
The International Standard Bible Encyclopedia
Endeavor
The International Standard Bible Encyclopedia, James Orr (1915). Public domain, from the CrossWire SWORD module.