Why is dmesg called dmesg?

I think it stands for "diagnostic messages", as per the older¹ man page (referenced here too).

dmesg - system diagnostic messages
Dmesg looks in a system buffer for recent kernel diagnostic messages and reproduces them on the standard output

One of the oldest references appears to be a man page revision by Kirk McKusick dating back from 1985.

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^{1: the link doesn't always work - no idea why... I'm attaching a screenshot though you should still be able to access the page via Google's cache.}

I think the dmesg command just stands for display messages. The FreeBSD manpages seem to share this view:

dmesg -- display the system message buffer