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Any extra person who fulfills a need in an existing group, especially musical, regardless of its actual size. Based on the famous conundrum of the original musical group The Beatles, "who was the fifth Beatle?"

Opposite of a "5th wheel".
Jake sits in with us on keys and fills the gap--he's totally our 5th Beatle.

Our group orgy was sort of ho-hum until Jan showed up; now she's our 5th Beatle!
by Bancis Franken October 11, 2013
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