The Java class and many other occurrences grew out of the name of the Observer/Observable design pattern[1] in the "Gang of Four" "Design Patterns" book.[2]
The Go4 call this a “Subject” rather than an “Observable”. As far as I can tell the only occurrence of the word “observable” in the Go4 book is:
> Other user interface toolkits that employ this [Observer] pattern are InterViews [LVC89], the Andrew Toolkit [P+88], and Unidraw [VL90]. InterViews defines Observer and Observable (for subjects) classes explicitly. Andrew calls them "view" and "data object," respectively. Unidraw splits graphical editor objects into View (for observers) and Subject parts.
1. https://en.wikipedia.org/wiki/Observer_pattern
2. https://en.wikipedia.org/wiki/Design_Patterns