This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The historical origin of 茟 is uncertain. It visually combines 艹 (grass) and 聿 (a writing brush or stylus), but the exact development is unclear.
With no clear meaning, picture a grass-like plant (艹) growing beside an old writing brush (聿). The image is of something obscure and hard to define.
For イツ, imagine an itsy-bitsy sprout (艹) poking out next to a brush (聿): itsy -> イツ, a tiny, uncertain thing.