A wristwatch with an EIA-232 interface. A clip fitted round the watch and made electrical contact. This clip had a socket for a stereo style jack lead the other end of which was a 25-way D-type connector. The lead allowed you to enter phone numbers etc. into the watch without having to play with tiny buttons. It also meant if the battery on your watch ran out you could restore the data without having to type it all in again.
It was around the era of the 8-bit home computers like the Spectrum, BBC Microcomputer, Apple II, C64 - the 1980s.