The unfortunate (but often hilarious) result of a busty woman fitting herself into a swimsuit too small for her body. The suit's tight fabric crushes her breasts together, creating a single, enormous boob in the center of her chest.
D00d, that chick's monoboob is outta control! She looks like a boobclops.
when a woman is so fat her boob get hidden by a huge roll of fat, creating the illusion of one long boob across her body. its a pretty nasty look.
girl: have you seen that new chemistry teacher? she has a total monoboob!