Extremely crowded; packed. A colloquial way to say a place is very full of people or vehicles.
週末の駅は激混みで、ホームに入るのも一苦労だった。
The station was packed on the weekend; it was hard just to get onto the platform.
セール初日は店内が激混みになるから、早めに行ったほうがいいよ。
On the first day of the sale the store gets extremely crowded, so it's better to go early.
お祭りの通りは屋台と人で激混みだった。
The festival street was packed with stalls and people.