A question of opposites

At a petrol station recently I saw a sign that said:

Warning: Highly inflammable

I’ve often wondered how or why this phrase came about. The prefix in generally means not or against, e.g. {in}ability, {in}capable, {in}excusable, {in}ert, {in}ept. Yet for some, probably historical, reason, flammable and inflammable mean the same thing, which is bonkers.

Taken at face value, the above statement is like saying “Watch out! This thing is highly unlikely to do anything at all.” So, a bit like a four-toed sloth then? A more accurate spelling might be enflammable, like {en}raged, {en}flamed, etc. But we’re stuck with the poor spelling.

This led to an interesting comparison of words that don’t have opposites in the pub the other night. Yes, because I’m so sad that I go to the pub to talk etymology:

• Today I’m quite gruntled — happy
• The weather is clement — a nice day
• Do you like my cognito outfit — it stands out from the crowd
• My computer is funct — it still works
• You’re looking rather sheveled there — smartly ironed clothes
• The terrain is quite dulating — rather flat

There are hundreds more, most of which are rather expertly woven into how I met my wife. No wonder English is so hard to teach.