All in a twitter; in a fright. Twittering is also the note of some small birds, such as the robin, &c.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
To Twit * TwittocNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes