A phrase that came into popular culture through a Saturday Night Live skit with Will Farrell.
In the skit, Farrell portrayed a member of the 80s rock band Blue Oyster Cult as they recorded their classic hit "Don't Fear the Reaper" in the studio.
Farrell played the cowbell quite forcefully on that track, much to the dismay of the rest of the band.
Yet surprisingly, the studio producer loved Farrell's cowbell playing, and demanded "more cowbell" on the track.
That was brilliant, man, the track needs more cowbell!