A doohickey is a small object whose name you have forgotten or do not know, especially a part of a machine — a knob, a clip, a bit of plastic that holds two pieces together. “There’s a little doohickey on the back” is the kind of thing a parent says while turning a new gadget over. It is American and informal. British speakers more often say doodah or whatsit for the same idea. It is not one specific tool; the whole point is that you cannot, or cannot be bothered to, use the real name. Though actually, it often sounds a bit more “mechanical” than doodah: people reach for doohickey when the thing belongs to a device. You would not usually call a person a doohickey, and you would not write the word in a repair manual if you knew the proper name. Once you know it is a screw, call it a screw.

那個小零件、那個小東西（美式口語，尤其是機器上叫不出名字的零件）