What does the word "symplectic" mean?

The term "symplectic group" was suggested in The Classical Groups: their invariants and representations (1939, p. 165) by Herman Weyl:

The name "complex group" formerly advocated by me in allusion to line complexes, as these are defined by the vanishing of antisymmetric bilinear forms, has become more and more embarrassing through collision with the word "complex" in the connotation of complex number. I therefore propose to replace it by the corresponding Greek adjective "symplectic." Dickson calls the group the "Abelian linear group" in homage to Abel who first studied it.

Take a look at the Earliest Known Uses of Some of the Words of Mathematics web page.

The following is from page 1 of Lectures on symplectic geometry, by Ana Cannas da Silva:

As a curiousity, note that two centuries ago the name symplectic did not exist. If you consult a major English dictionary, you are likely to find that symplectic is the name of a bone in a fish's head. However ... the word symplectic in mathematics was coined by Weyl who substituted the Latin root in
complex by the corresponding Greek root, in order to label the symplectic group.

There is a brief explanation here. It looks like the term was coined by Weyl, and was a result of modifying the Latin prefix “com-” from “complex” to the equivalent Greek prefix “sym-”. This is a pretty obscure way to coin a word if you ask me!