Meaning of ERR_LIFETIME_EXP, please ?

Apologies if this is an obvious question, or one answered elsewhere, but
what does the following error reply in my access.log file mean ?

131.251.6.89 - - [27/Jun/1996:17:30:34 +0100] "GET
http://www.w3.org/hypertext/WWW/MarkUp/Wilbur/" ERR_LIFETIME_EXP 774

I had quite a few of these from squid 1.0b17 on Linux 1.2.8: none since I
upgraded to squid 1.0.0 this morning, but that may be because I haven't
used the cache much today. They seem to all be for pages which I would
expect to get from my parent cache.

In all cases the requested page was accessible if caching was turned off,
but simply pressing reload gave the same response. The response to the
browser was a DNS timeout.

Many thanks,
Andrew

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