The Complete Guide to Anachronisms in Samurai Champloo--
Episode 10: Lethal Lunacy:
Silent_Edge from Swords & Sunflowers suggested that this member of the crew Shoryuu assaults on the bridge may be giving something like a gang sign:
Momoi Seishirou. This would-be hotshot swordsman claims to be so fast that he's known as "human electricity" (rendered by his VA in Japlish exactly that way, but in some translations given, less anachronistically, as "human lightning").
Not only is Seishirou not nearly fast enough to be called anything of the sort (in fear of Mugen he does admit he made it up himself), but, though it's not impossible, it's not too likely that he'd use this terminology.
The word "electricity" was coined in England in 1600, but there was no electrical anything in Japan until long after that.
(Electricity was first used in Japan on March 25, 1878 at the Institute of Technology in Toranomon, Tokyo when an arc lamp was switched on in commemoration of the opening of the Central Telegraph Office.)
Still, it isn't impossible that he would have heard the term--there was contact between the UK and Japan through the Ryuukyuus from 1613 until the 1630s--so I'll give him the benefit of the doubt. --Thanks again to Silent_Edge and also to dakameleon for raising and settling this question. =)
Anachronisms Occurring Only in the English Dub:
More on Seishirou! Andrew Thivyanathan sends the folowing well-researched note:
"...I thought that you might want to make a note that in the Geneon English dub, Momoi Seishirou calls himself a 'human dynamo' rather than 'human electricity' or 'lightning.' This doesn't really make things any better because (according to http://www.etymonline.com} the word was short for the German word 'dynamoelektrischemaschine' which itself was word created by a German engineer in 1882 to name his generator. Of course, using the word to refer to an energetic person could have only begun after that. The German engineer's word is itself inspired from the Greek word 'dynamis' meaning 'power,' but all references state that the English word 'dynamo' came from the long German word, not 'dynamis.' "