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1.
someone who may be normal or boring.

nothing is really going on for them. physically just average features not ugly or pretty.
mentally just not exciting to talk to, dont do much cool stuff.
"that girl is pretty!"guy 1
"nooo! she is sucha plain jane" guy 2
"well what girl is pretty to youu, around here?" guy 1
" that one shes diffrent, see those clothes look vinatge-y. and she has a diffrent interesting nose! pretty!" guy 2
"yeah i guess your right, who wants a plain girl?" guy 1
by sexy_girl92 November 04, 2006
 
2.
The best flavor of any tasty treat.
I love plain M&Ms.

I love plain jellybeans.
by Kevin21boston November 12, 2006
 
3.
A small village near Airdrie in North Lanarkshire, Scotland.

Look up "Airdrie" on urbandictionary.com, Plains is mostly similar.
Although Plains now has a train line running through it, unlike Airdrie and nearby village of Caldercruix it does not have a station. This is entirely due to the decision made by Network Rail against Plains.
However, due to tickets on the Airdrie>Bathgate line being ridiculously expensive, very few people are actually using the railway so Network Rail are getting their comeuppance.
Plains is pure the bomb man! I'm gutted they don't have their own train station!
by Plains, Trains & Automobiles January 23, 2011