A state of extreme crowding — packed like sardines; very crowded so there's little room to move.
ラッシュアワーの電車はいつもすし詰めだ。
Rush-hour trains are always packed like sardines.
会場は立ち見で、客はすし詰めになっていた。
The venue was standing-room only and the audience was jam-packed.
荷物を詰め込みすぎるとトランクがすし詰めになる。
If you cram too much luggage, the trunk becomes completely packed.