In general, a watchdog timer is a piece of code (or external hardware) that attempts to keep some other piece of code (or hardware) from getting stuck in an endless loop. Usually that's done by some common data structure that the thing being watched is supposed to update frequently. The watchdog keeps checking; if it sees that the data has not been updated, it assumes the other thing has gone awry and does something.
In many cases, "something" is a panic, a forced shutdown or a reboot, because the thing being watched is critical and nobody can imagine why it would have stopped unless all hell has broken loose somewhere.
A Google search for "watchdog timer panic" will turn up plenty of examples. On old SCO systems, this is apt to indicate a problem with the Intel Pro100 nic card driver; see https://aplawrence.com/cgi-bin/ta.pl?arg=109718.
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