Featherweight Java (http://www.fos.kuis.kyoto-u.ac.jp/~igarashi/papers/fj.html) was a seminal paper which showed type soundness for Java. The formal syntax was a subset of Java subset and preserved the interesting features in the semantics (as opposed to encoding a language in an extension of the lambda calculus, which is an alternative style of formalisation). So, "featherweight flavour" refers to formal type systems work which follows this style of formalisation.