jam-packed; teeming; heaving with people
Idiomatic simile describing a place so crowded that people are packed together like potatoes being washed in a tub. Often used with 混雑する or 混んでいる.
The beach in summer was jam-packed with people.
通勤ラッシュの電車はまさに芋を洗うようだ。
The rush-hour train is exactly like a can of sardines.
Literally 'like washing potatoes'. The image comes from the way potatoes are jostled together when washed in a tub, evoking a tightly packed, jostling crowd.