jam-packed; teeming; heaving with people
Idiomatic simile describing a crowd so dense that people are jostling against each other, like potatoes being washed in a tub. Often used with 混雑, 人混み, or similar words.
See also: 芋を洗うよう
The summer festival venue was jam-packed with people jostling against one another.
通勤ラッシュの電車はまさに芋の子を洗うようだ。
The rush-hour train is exactly like a scene of people jostling together.
From the image of washing a large number of small potatoes in a tub, where they bump and jostle against each other. The exact origin is uncertain, but it is a traditional Japanese simile for a crowded, jostling scene.