Describes a space or container so crowded that people or things are pressed tightly together, like sushi rice packed into a box. Often used for trains, elevators, or small rooms.
ぎゅうぎゅう is a mimetic word for being tightly packed, often used for objects or spaces. すし詰め is more vivid and specifically evokes the image of sushi rice packed in a box.
Etymology
From 寿司 (sushi) + 詰め (packing). The image comes from sushi rice being firmly pressed into a box or mold, likened to people or things packed tightly together.
