I came across it in James Joyce's Ulysses, where (in the first few pages before I gave up again) Steven Dedalus and Buck Mulligan are staying in a Martello tower (originally built as a defence against possible Napoleonic invasion) and Joyce says: "Two shafts of soft daylight fell across the flagged floor from the high barbacans."
The principal meaning of “barbican” given by the Oxford English Dictionary (2nd Edition) is an outer fortification or defence to a city or castle, especially a double tower erected over a gate or bridge; often made strong and lofty, and serving as a watch tower. Examples they give of this usage are: “The Erle made … bulwerkes and barbycanys atwene the Toure and the cytie”, Fabian 1494; and “The usual entrance .. over which he had erected a gate-house or barbican,” Walter Scott, Kenilworth, 1821.
There is no doubt that “barbican” came to have such a meaning, and there is little doubt how the name came attached to the area. The Roman fort which became part of the City wall had towers on either side of the gate, which was later called Cripplegate. But there are competing claims from as far away as Arabia and Persia for the origin of the word itself.
They suggest that Barbican perhaps came from combining two Persian words barbar khanah to mean “house on the wall”, but say they can’t find any examples of such a use. Not willing to give up on an exotic lineage, they also suggest that the Arabic word barbakh, meaning a canal or channel through which water flows, might have been the source of the loop-hole meaning. Finally, and still firmly in the Middle East, they suggest the Arabic or Persian bab-khanah, meaning gate house, which was regularly used for a towered gateway. However, they concede that it is difficult to get from that to a Latin form of the word.
In fact, there is a source for “barbican” much closer to home. In his Survey of London published in 1598, John Stow writes: “On the west side of the Red Cross is a street called the Barbican, because sometime there stood on the north side thereof a burgh-kenning, or watch tower of the City, called in some languages a barbican.”
This line was continued by Sir Henry Spelman in the 1640s. He explained the name as a combination of “burgh” meaning tower; and “ken” meaning see or watch (as in the folk song “D’ya ken John Peel”).
At the same time, there was some support for the Arabic derivation. In his Britannia (1586), William Camden referred to “an Arabick name Barbican” for a watch tower or military fort.
I would be interested to receive any alternative derivations