Bolt: A strong pin, of iron or other material, used to fasten or hold something in place, often having a head at one end and screw thread cut upon the other end.
The term bolt-in was originally coined, to the best of my knowledge, by my friends and former colleagues Leigh and Scott, who have a software consultancy based in Sydney, Australia. Generally speaking, bolt-ins “bolt in” significant enhancing functionality around existing classes. This sounds similar to the concept of veneers, and indeed bolt-ins can be considered a closely related concept. However, they are a whole lot more substantive, and they have fewer constraints than veneers.