bucket有“吊桶”的意思，come down in bucket雨下得如同一桶桶水倒下来，不就是倾盆大雨么。

【例句】
It started coming down in bucket just as I began cycling home.
当我骑自行车动身回家时，下起了倾盆大雨。

The road was flooded yesterday because the rain was coming down in buckets.
昨天下了一场倾盆大雨，路面都被淹了。