to blow, exhale
龡 centers on the act of blowing air out, especially in a musical context: to blow a wind instrument or simply exhale.
龡 combines 龠 (a panpipe or wind instrument) with 欠 (a person with mouth open, often associated with exhaling or yawning), suggesting blowing into a musical instrument.
The left side 龠 looks like a bundle of vertical pipes tied together—a wind instrument. The right side 欠 shows a person with an open mouth. Together, they show someone blowing air into the pipes to make music.
For スイ, imagine blowing so hard that you start to sweat: sweat -> スイ, and the air rushes out of your mouth into the instrument.