虽

Bottom 虫 — the indexing radical. Originally 雖 named a small lizard or insect; the 'although' meaning was a phonetic loan that took over. The radical pins this character to its biological origin even though no learner today thinks of bugs when reading 虽然. A reminder that grammar words often started life as concrete nouns.

Top 口 — originally part of the phonetic 唯 wéi that supplied the sound for older 雖. The 1956 simplification kept only the 口 plus bottom 虫, dropping the middle 隹 (bird). So 口 here is a phonetic residue more than a real 'mouth'; the yi/sui phonetic link is now invisible without history.