The scuffle forced the plane to be landed in Chicago, where the warring passengers were both evicted.
The mid-air fight started because the man, seated in a middle seat, used a "Knee Defender" - a USD 21.95 gadget that attaches to a passenger's tray table and stops the person in front of them from reclining.
The hot-headed female sitting in front of him then then stood up, whirled around and tossed a cup of water at her tormentor, authorities were quoted as saying by the New York Post.
That is when United decided to make the unscheduled stop at Chicago's O'Hare Airport, where the flight was met by Chicago cops and Transportation Security Administration officials.
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They questioned the unidentified combatants - both 48 - but did not place either under arrest, determining that the dust up was a customer service issue and not a criminal matter.
Ironically, both passengers were sitting in United's Economy Plus section, which is advertised as having four more inches of legroom than the rest of the plane.