引っ込む
ひっこむ · hikkomu · JLPT N2 · verb-godan
to cave in
引 (pull) + 込む (go into): pulled back inside, so it sinks or draws back
引っ込む is 引く (to pull) plus 込む (to enter or crowd in). Something pulled back inside itself draws back, sinks, or caves in.
引 — bow 弓 + vertical stroke 丨
A compound ideograph showing a bow 弓 with a vertical line 丨 representing the drawn bowstring. The original meaning was 'to draw a bow,' which extended to 'pull,' 'lead,' 'guide,' and 'cite.' The vertical stroke depicts the action of pulling the string back.