na-adjective, noun
great haste; mad rush; panic
Describes a state of extreme hurry or fluster, often with a sense of panic or disarray. Used as a noun or adjectival noun (na-adjective).
寝坊して大慌てで家を出た。
I overslept and left the house in a mad rush.
火事の知らせに、みんな大慌てだった。
Everyone was in a panic at the news of the fire.
慌てる is the verb 'to panic, to be flustered'; 大慌て is a noun/adjectival noun emphasizing the degree of haste or panic.
Compound of 大 (おお, 'big, great') and 慌て (あわて, 'haste, panic'), the continuative form of 慌てる.