Sub-ether signalling device found in Douglas Adams' Hitch-hiker's Guide To The Galaxy book trilogy. The Electronic Thumb is a considerably more advanced version of the device that terrestrial hitch-hikers used to hitch lifts from cars. In the books, Ford Prefect (and, presumably, other interstellar hitch-hikers) used the Thumb to flag down and hitch rides from flying saucers.
The Thumb isn't entirely benign - it can actually hold a small amount of control over the ship being flagged down. This is evident in So Long, And Thanks For All The Fish when a departing flying saucer is prevented from taking off and forced to land again by signals from the Thumb. As a result it's not a particularly popular device among strags (strag: non-hitch-hiker), and half the electronic engineers in the Galaxy are constantly trying to find fresh ways of jamming these signals. (Meanwhile, the other half are trying to find ways of jamming the jamming signals. In the books, evidence suggests the latter are winning.)
The Thumb is described as "a short squat black rod, smooth and matt with a couple of flat switches and dials at one end". It also has a few winking lights on it.