Originated as a combination of the english words "either" and "yes." It is used to sway the decision towards one perticular decision. When giving someone a choice if you want them to choose one perticular option you would use the word yeither.
There are two roads going through town, you can take yeither. (usually used in conjection with non-verbal communication)
ayon kay G'stein ika-20 ng Marso, 2008