A doodah is a small object when you cannot remember its name, or never knew it. “Pass me that doodah” means “that thing — you know the one.” It is British and informal, the sort of word you say at home, in a kitchen, or in a shop, not in an exam essay. Any small object can be a doodah for a moment, until the real name comes back. Though actually, British English has a little family of these nameless-thing words — doodah, thingummy, whatsit — and they all do the same job. Americans more often say doohickey or doodad. Doodah is not a special type of tool; the whole point is that the name is missing. There is another, older British phrase, all of a doodah, meaning very excited or in a flap, but that is a different use. Here the word is only the little unnamed object.