Polk's nickname, "Young Hickory", refers to his status as Andrew Jackson's standardbearer in the 1845 election.   Throughout his political career, Polk allied himself very closely with Andrew Jackson's policies.  Although he was less flamboyant than his predessor, Polk arguably was one of the most hard-working Presidents to have served in office.  During his time in office, the US became a continental power with the annexation of Texas and California to the Union. 

Perhaps "Polk the plodder" was even more "hickory" than Jackson -  pale, tough, hard, resilient, and shock resistent.

As for "young": Though in some way Polk's conduct was "adult" to the extreme, and though he already looked like an old man in his early fifties,  his diary reveals that there was something "teen-age" about him until he died. It's a kind of "serious college-boy"-naivity that touched me deeply.