From Longman Dictionary of Contemporary Englishnot leastnot leastformal used to emphasize that something is important My mother was upset about his appearance here, not least because she felt it was invading her privacy. → least
Examples from the Corpus
not least• Nevertheless it requires separate assessment, not least because it drew on certain areas of experience not directly dominated by the monarch.• I didn't relish this: not least because it meant that I didn't break my silence until the cheese course.• Alex Ferguson will have loved this, not least because questions had started to be asked of his team and star names.• There are many factors which limit productivity; not least is employee education.• And it annoyed her intensely, not least of all because she still felt a long way from figuring him out!• Like all good music it speaks of love, not least of the medium itself.• And last but not least, the baby of the family. 