I see a comment on YouTube says:
This dropped as I was studying maths
It's about a YouTuber says she doesn't like math. So what does this sentence mean?
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The expression This/it/something dropped is most frequently used in two contexts.
The first is to say that I suddenly understood or had a flash of inspiration. The usual expression is that the penny dropped, arising from the use of coin machines where the dropping of a coin produces a product or result. A similar expression is the light dawned.
This/it/something dropped is also used to describe mail or packages that are thrust through the aperture in many front doors intended for mail (and known by sundry names including letter box, letter plate, letter hole, mail slot or mailbox).
The expression may refer to one or other although it's impossible to say without further context.
Dropped here means was published. So, the video was published while the commenter was studying math.
This is the relevant definition in Urban Dictionary:
The availability of a product or service to the general public. The term is most often used in relation to the release of a music album, a DVD video release, or hardware or software, but can be used for other products (i.e., concert tickets.)
Windows 7 is slated to drop on October 22nd
The new iPods dropped last week.
This slang has been in use for quite some time, specifically when referring to music being released. According to the OED in this sense it dates back to 1988.