Cost accounting question...?

1)what is meant by semi variable cost?
2)what is meant by sunk cost?
3) in cost accounting when working with direct labor cost how do you treat idle time, over time and premium time?

Semi variable costs do vary with volume but not in a direct way. Things like certain kinds of tools, machine space, supervisor costs will increase in a stepped way rather than uniformly with increased production.

Sunk cost is the inherent setting up cost in an operation which has to be recovered before the contribution from that operation can be seen to be a clear profit.

The third question is a whole separate class of questions in its own right. There are different opinions on what ios best and a lot depends on the processes you are costing. For each of these, you can estimate an inherent amount from historical averages and factor these into the base costs and then account for variances on experienced costs. Or especially if you charge extra for rush work or non-standard time work you can use the overtime and premiun time as costs against this added contributions.

What you must be careful of is making embedded assumptions that distort the base costs when there is substantially greater or lesser work than usual in the operation. Many people now prefer the casdading pool system of costings where contributions are passed in more and more central pools and certain costs like these labour specialities are mopped up at the end.