Extremely crowded; packed to the point that there is little or no room. Colloquial use with "超-" to emphasize.
朝の電車は今日も超満員で、ドアが閉まるときに押されて大変だった。
The morning train was packed again today; it was rough being pushed when the doors closed.
ライブ会場は超満員で、入場制限がかかった。
The concert venue was packed, and entry was restricted.
行列ができていて店内は超満員だから、少し時間をずらした方がいいよ。
There’s a line and the store is packed, so it’s better to go at a different time.