What remains and keeps returning after the first exchange is done. A share that continues rather than concludes.
Latin residuus (remaining, left over), from residere (to remain behind, to settle): re- (back) + sedere (to sit); a residual is what stays sitting after the first exchange has already moved on.
What remains and keeps returning after the first exchange is done. The root's sitting-behind is a fitting image for a share that continues rather than concludes.