This election produced an unexpected newcomer winner.
Tips
memory
爆 (bào) = to burst/explode, 冷 (lěng) = cold, here meaning 'a long shot.' Together: a long-shot result 'explodes' onto the scene. Originally betting/sports slang, now general-purpose.
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Most common in sports headlines and news (爆冷门 bàolěngmén is a near-synonym), but you'll also hear it about elections, awards shows, and any contest where favorites get upset.