Wikipedia:TROUT

Trout slapping originated in 1995 with internet relay chat (IRC). While in an IRC chat room, the IRC client mIRC would allow users to enter the command:

which would trigger the chat room to announce something along the lines of:

The trout as the standard weapon-of-choice for slapping users in IRC can probably be attributed to the surreal humor that computer programmers are known to appreciate. There is additionally an old saying that some personal experience is "better than a slap in the face with a wet fish." A 1971 Monty Python sketch called The Fish-Slapping Dance may be partly to blame, even though this sketch did not specifically involve trout (sardines and a halibut).

Today, getting whacked with a wet trout can be compared to when your mother said she was "hitting you over the head with a wet noodle", and it makes about as much sense. When someone does something inadvisable that they had the experience and intelligence to avoid, you may likely see the suggestion that they be "trouted" — or "trouts all around", as it has recently become more stylish to blame all parties in a dispute instead of one.

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Wikipedia