[common; from ham radio]
1. Like doorstop but more severe; implies that the offending hardware is irreversibly dead or useless. “That was a working motherboard once. One lightning strike later, instant boat anchor!”
2. A person who just takes up space.
3. Obsolete but still working hardware, especially when used of an old, bulky, quirky system; originally a term of annoyance, but became more and more affectionate as the hardware became more and more obsolete.
Auctioneers use this term for a large, undesirable object such as a washing machine; actual boating enthusiasts, however, use “mooring anchor” for frustrating (not actually useless) equipment.