Other forms: combustibles; combustibly
Combustible means "able to catch fire," like matches or the extremely dry forest floors that people sometimes forget are combustible — until they go up in flames.
When the adjective combustible first entered the English language in the sixteenth century, it was used to describe things that burn easily. The word soon came to apply not only to objects that ignite but also to tempers that are prone to "catch fire" easily. If you’ve ever been around a person who has a combustible temper, you can understand the connection.