means A vegetable (often a cucumber) preserved in brine or vinegar — or, by extension, a tricky predicament you've gotten yourself into.

from From the Dutch 'pekel,' meaning a salty brine for preserving food, which slid into English in the late Middle Ages. The food sense came first; the 'in a pickle' sense — being stuck in a sour, briny mess — followed naturally, since a thing in pickle is well and truly soaked in trouble. Shakespeare uses the predicament sense in 'The Tempest,' so the metaphor was already bobbing around by the early 1600s.