The term is pronounced Knickerboxer, and it is also spelled Knikkerbakker, Knickerbakker, Knickerbacker, Knickerbocker. The word begins with Harmen Jansen Knickerbocker, a Dutch colonist in New Netherland (New York).

In 1809, Washington Irving published a satire called "A History of New York" under the pseudonym "Diedrich Knickerbocker". In the early days it referred to Manhattan's aristocracy in the early days.

Much later than its origin or its use as Irving's pseudonym, it was the New York Knicks' old team name.

NOTE: In some areas of the USA, people pronounce Knickerbocker just as it looks, as if saying one of these words "bock, lock, sock".

As far as "knickerbockered", I could find no reference. However, the "-ed" changes the meaning into a verb, so it may refer to the 1922 blizzard, called the "Knickerbocker Storm" which knocked down the Knickerbocker Theater in Washington DC. If this is the reference, I'm guessing that to be "knickerbockered" might mean to be "stormed" or to be "knocked down".

Harmen Hals died in 1669.

Harmen Hals was born in 1611.

Harmen van Bol'es died in 1764.

Harmen van Bol'es was born in 1689.