The first apparent coinage of "simplicitude" appears to be as early as 1985, although precise definitions are scarce. Therefore, I can't take credit for coining the term, although I came up with it without knowledge of it's previous uses. I first used the term in late 2000 in conjunction with a musical form I've developed, and as such, to my knowledge, I've provided the first definition, as follows:
"Simplicitude is a musical form that explores the natural tensions, conflicts, and contrasts that form the basis of understanding for any human experience. For example, joy without sorrow would be hollow; Love without justice would be trite; Relationship without vulnerability would be worthless."
The original concept was a combination of simplicity combined with etude (a term connoting a musical study). So, in one sense, simplicitude can mean a study in simplicity. As with many words, "simplicitude" can be applied in various contexts, so my definition within a musical context need not constrain its usage. It certainly could take on broader literary application.
A more complete discussion along with examples of Simplicitudes are at dbeckett.com.