Even the slickest cowboys find themselves in a "fish" from time to
time. There's usually somethin in the air when they do.  what is it?

Hello.

When a cowboy is in a "fish," it's because rain is in the air.

A "fish" is cowboy slang for an oilskin slicker that you wear when it's raining.

sources:

"Fish - A Slicker, named after "Fish" brand slickers."
source: Cowboy Definitions and Lingo
http://www.theoldwestwebride.com/txt2/def_lingo.html

'Fish ~ a cowboy's rain slicker, from a rain gear manufacturer whose
trademark was a fish logo. "We told him it looked like rain, but left
his fish in the wagon anyhow."'
source: Old West Slang and Phrases
http://freepages.genealogy.rootsweb.com/~poindexterfamily/OldWestSlang.html

"fish - Cowboy's slicker or raincoat."
source: DJ's Texas State of Mind: Old West Slang
http://djstexasstateofmind.homestead.com/OldWestSlang.html

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search strategy:
"cowboy slang", fish
"cowboy slang", fish, slicker, rain

I hope this helps.