Colonic means “of the colon” — and this colon is the large intestine, the last long tube of your gut. It is not the punctuation mark :, and it is not a colony. Doctors use it as a quiet, technical adjective: colonic disease, colonic tissue. As a noun, a colonic is a treatment that washes out the large intestine (also called colonic irrigation). Some spas sell this as a clean-out; many doctors are not impressed and say a healthy person does not need it, and it can even be unsafe. So the noun feels like wellness-and-advertising talk, while the adjective feels like hospital talk. It is not a daily word. You do not need it to talk about a stomach ache in ordinary English. Because it looks a little like colonial, students sometimes try to hang it on the colony family. Though actually, it hangs on a different colon: the body part, from a Greek word for that stretch of gut.