1. Slang term for a pair of breast implants that look unnatural, rock-hard and "bolted" onto their owner's chest much like the stiff, painful looking screws that were bolted on to the side of Frankenstein's neck in the classic horror film. This "bolted-on" look typically happens because of what is called capsular contraction. Capsular contraction is when scar tissue forms in the chest cavity and tightens around the implant(s), making the breast(s) hard and unnatural looking. The "bolted-on" look can also occur when extremely large breast implants are inserted into someone without enough fatty tissue in the breast area. "Bolt-ons" do not look natural. All bad boob jobs where it's obvious that the person has implants can be referred to as "bolt-ons".
2. A messed up boob job.
3. Cause for a malpractice suit.
"That chick's boob job doesn't look so good. It looks like she's got two rocks in her shirt."
"Yeah, those are a serious pair of bolt-ons."