flute
竾 centers on a musical wind instrument: a flute, typically made from bamboo.
竾 combines 竹 (bamboo) with 也, which likely serves as a phonetic component. The character is a rare variant of 笛, both meaning flute.
Bamboo 竹 on top suggests a bamboo instrument, and the lower part 也 looks like a mouthpiece and finger holes. Together they make a flute.
For チ, imagine a flute playing a cheerful tune: the first sound 'chee' in cheerful cues チ, and the bamboo flute makes the music.