It’s one of those Ye Olde English artifacts that the language is (in)famous for:
The in- of incombustible is a common prefix meaning “not,” but the in- of inflammable is a different prefix. Inflammable, which dates back to 1605, descends from Latin inflammare (“to inflame”), itself from in- (here meaning “in” or “into”) plus flammare (“to flame”). Flammable also comes from flammare but didn’t enter English until 1813. In the early 20th century, firefighters worried that people might think inflammable meant “not able to catch fire,” so they adopted flammable and nonflammable as official safety labels and encouraged their use to prevent confusion.
_from Inflammable Definition & Meaning - Merriam-Webster
Personally, if I see the word ‘inflammable’ I parse that as “tends to catch on fire at the drop of a hat; use extra caution.”
