"Not seldom" is an expression that means "not often" or "rarely." It is a useful phrase when you want to convey the idea that something happens occasionally or infrequently. If you are looking for synonyms for "not seldom," you might consider using expressions such as "not frequently," "infrequently," "occasionally," "sometimes," "at times," "now and then," "on occasion," "from time to time," and "once in a while." These synonyms will help you to vary your language and express the same idea in different ways, making your writing or speech more interesting and engaging.