What is the root word for collapsible

It is formed from the word "collapse". This root word comes from the Latin collāpsus, "fallen together".

Collapsible is formed from the verb collapse, which comes from the Latin collāpsus, "fallen together." This makes sense, since something that is collapsible "falls" into a tighter, more compact size or shape. Of course, anything that is able to fall can be considered collapsible, but the word refers more to something that is intentionally made to fall, rather than something like a rickety building that might collapse.