Centuries before plastic as we know it was invented, the word plastic referred to anything that could be molded. However, when it was invented in 1855, it was called parkesine, and the modern definition wasn't applied to plastic until 1909. Okay, so the word was originally borrowed into the English language in the 1630s from the Latin word plasticus, which meant "of molding". This in turn comes from the Ancient Greek word plastikos, from plastos, "molded", which can be conjugated to plassein, a verb meaning "to mold" (as you can see, there isn't much semantic change going on here). Plassein also gave us our word plasma, through the same word in Greek and Latin, under a connection of something molding like plasma. This derives from Proto-Hellenic platto, from Proto-Indo-European pele, meaning "to spread", as in you spread material to mold it. Usage of both the words plastic and plasma followed roughly the same trend in literature, peaking in the '80s, at about the same amount, as well.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.