1. A one-sided fudge factor, that is, an allowance for error but in only one of two directions. For example, if you need a piece of wire 10 feet long and have to guess when you cut it, you make very sure to cut it too long, by a large amount if necessary, rather than too short by even a little bit, because you can always cut off the slop but you can't paste it back on again. When discrete quantities are involved, slop is often introduced to avoid the possibility of being on the losing side of a fencepost error.
2. The percentage of ‘extra’ code generated by a compiler over the size of equivalent assembler code produced by hand-hacking; i.e., the space (or maybe time) you lose because you didn't do it yourself. This number is often used as a measure of the goodness of a compiler; slop below 5% is very good, and 10% is usually acceptable. With modern compiler technology, esp. on RISC machines, the compiler's slop may actually be negative; that is, humans may be unable to generate code as good. This is one of the reasons assembler programming is no longer common.