This phrase was originally used by Christopher Walken on SNL with Will Ferrel. He was indicating that something needs more emphasis, or literally "Needs more cowbell".
Aired on April 8th, 2000.

This is also referenced in World of Warcraft in a quest in Outland, where you escort a ghost cow.
That was pretty good but it needs more cowbell.
by twimoon December 11, 2015
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