[1] cf. Phrynichus the Atticist 362, whose comments are worth quoting in full: "Some refer to the sthqu/nion ['breasticle'] of a bird, but not properly. If you need to use a diminutive, [say] sthqi/dion ['breastlet']. If it is not a diminutive, whence did this evil also come into the language of the Greeks?" Of sthqu/nion and sthqi/dion, the former is much better attested, occurring, for instance, multiple times in the Septuagint (e.g. Exodus 29:26, Leviticus 9:20) in reference to the breasts of sacrificial animals. Cf. Pollux 2.162 where sthqu/nion is defined as the middle of the breast. Adler also cites Lexicon Ambrosianum 819.
[2] This form, presumably a variant of the foregoing words, is unattested elsewhere.