I don't know about weirdest, but here are some quirky words:

• inflammable means the same thing as flammable

• "the/a". If you're a native English speaker, like me, it probably doesn't look unusual. I was listening to a lecture series on linguistics and it wasn't until then that I learned that most languages out there don't have a mandatory definite/indefinite article. In most languages, if you want to say "cat", you can say "cat". English requires you to say "a cat" or "the cat" -- the presence of an article to indicate whether the thing you're talking about is unique or not. That's an unusual feature for a language to have. It's baked into how I think, but a lot of the world just doesn't work that way.

  https://en.wikipedia.org/wiki/Article_(grammar)#Crosslinguistic_variation

  Articles are found in many Indo-European languages, Semitic languages (only the definite article)[citation needed], and Polynesian languages; however, they are formally absent from many of the world's major languages including Chinese, Japanese, Korean, Mongolian, many Turkic languages (including Tatar, Bashkir, Tuvan and Chuvash), many Uralic languages (incl. Finnic[a] and Saami languages), Hindi-Urdu, Punjabi, Tamil, the Baltic languages, the majority of Slavic languages, the Bantu languages (incl. Swahili). In some languages that do have articles, such as some North Caucasian languages, the use of articles is optional; however, in others like English and German it is mandatory in all cases.

• "data". It used to normally be the plural of datum, but within living memory has normally become a mass noun, like "water" or "air" or "love". It's not the only word to do this, but it's unusual.

• "deer". It's not the only word to do this either, but it's one of a small number of words in English where the plural and singular form can be (and traditionally, needed to be) identical. Today, it looks like regular forms of these are increasingly being considered acceptable, at least in American English ("deers", "fishes", etc).