Short for 'arcology', itself a word invented by architect Paolo Soleri as a combination of the words 'architecture' and 'ecology' and which refers to the theory of building large, self-contained structures for human habitation. 'Arco' is usually used in the context of SimCity 2000, a computer game in which the player can build arcologies, which are enormous, odd-looking, highly expensive structures that can contain thousands of people inside them. Three of the four arcologies in SimCity 2000 (the plymouth arcology, forest arcology and launch arcology) are often named using the word 'arco' in place of 'arcology', while the fourth simply adds the letter 'd' to the word 'arco' to form 'darco' (which the game states is an acronym for Deurbanized ARcological COnstruct).
For arcologies in general:
Getting to the arcos is easy, all you have to do is make a flat city, turn disasters off and then build.
For a specific type of arcology:
I don't make forest arcos, they don't have enough population to be worth it. I always go for launch arcos instead.
Short for asshole, in that arco's logo resembles your a-hole.
"You fucking Arco..."