注す
さす · sasu · JLPT N3 · verb-godan
to serve
🍶 さす sounds like sauce, and sauce is something you pour
The kanji 注 is 氵 (water) + 主 (host, master): the host pours a drink for the guests. The same kanji appears in 注ぐ (そそぐ, to pour). 注す is mostly used for pouring sake or tea into someone's cup.
注 — water 氵 + master 主 (phonetic)
A semantic-phonetic compound: 氵 (water) gives the meaning of pouring or flowing, while 主 supplies the sound. The original sense was 'to pour water in,' which extended to 'concentrate, focus' (as in pouring attention) and 'to annotate' (pouring explanation into a text).