Probably にんまりと (jisho.org) “smile complacently”. と “quotes” mimetic adverbs, and as I’ve mentioned elsewhere, if you see 〜りと it’s very likely to be a mimetic adverb. After that, you just need to guess. Here’s how I guessed, if you’re interested:
- nimari(to) sounds a tad bit irregular because it’s 3-mora long, whereas most of those are 4-mora long, so might be missing one mora;
- if it’s missing something, it’s probably a sound duplication/change somewhere, and it’s probably between the first and second mora (because ri is not really part of the mimetic “sound”, and I don’t expect the first mora to be deleted, but it’s just intuition);
- so I guessed it must be a dup of m- (from ma), so nimmari, or にんまり in kana.
P.S.: Obviously, it’s not a science; if you look at my other post, you’ll see I discuss a 3-mora mimetic word, so definitely it could be にまにまと or something, so sometimes you’ll guess wrong a few times before guessing right. I just got lucky. 