I think his point is that usually when you slap "in", "im", or "un" onto the front of a root word, it ends up creating an antonym of the shorter version: immaterial, inconsequential, incredible, inarticulate, untangle, unearth, etc. Intuitively it seems like "inflammable" should mean "not flammable", etc. Instead, "inflammable" is kind of a pointless, redundant word. Just kind of a linguistic oddity.
2/19/14 at 3:55pm