Old English piþa "central cylinder of the stems of plants," also, figuratively, "essential part, quintessence, condensed substance," from West Germanic *pithan- (source also of Middle Dutch pitte, Dutch pit, East Frisian pit), a Low German root of uncertain origin. Figurative sense of "energy, concentrated force, closeness and vigor of thought and style" is by 1520s. The pith helmet (1889, earlier pith hat, 1884) was so called because it is made from the dried pith of the Bengal spongewood.
1805, "to kill by piercing the spinal cord," from pith (n.). By 1852 as "remove or extract the pith from." Related: Pithed; pithing.