Lamo is a common typo that occurs when you try to type "Lmao" (which means laughing my ass off, for those of you who are not familiar with that, but seriously that's really lame and almost everyone knows what lmao means so if you don't just crawl into a corner and suck your thumb. i'm not kidding.) so anyway instead of writing lmao you write lamo and your friends just start laughing at you for writing lamo. it's pronounced "lah-moo" but that's all up for interpretation
in an instant messaging format:
amy: jeez that guy was like a shit salesman with a mouthful of samples
amy: lamo? hahahahha you said lamo.....