Chaff is the husks of corn separated from the grain by threshing. Be caught with chaff has been used since the late 15th century as metaphor for being easily fooled or trapped.
Related Idioms :
separate the wheat from the chaff = sort the wheat from the chaff
distinguish valuable people or things from worthless ones.
Chaff is the husks of corn or other seed separated out when the grain is winnowed or threshed. The metaphorical contrast between wheat and chaff is drawn in several passages in the Bible….for example in Matthew 3 : 12 : he will thoroughly purge his floor and gather his wheat into the garner….but he will burn up the chaff with unquenchable fire.