Abarim Publications' online Biblical Greek Dictionary
The noun θωραξ (thorax) means thorax (same word): the upper torso of man and beast, and more specifically, any kind of clothing that wraps and protects the torso, from a corselet to a short jacket and ultimately a piece of metal armor: a coat of mail, a wearable metal shield. This word is often translated with breastplate, but it should be noted that it mostly refers to a so-called cuirass, a front-and-back metal jacket that shields the entire upper body from attacks from all sides.
In a figurative sense, the θωραξ (thorax) is associated to one's breathing, spirit and reason (see πνευμα, pneuma), and is the counterpart of the οσφυς (osphus), loin, where the lower emotions and instincts were seated.
The Hebrew word for torso is חיק (heq), which comes from a verb that means to embrace or to draw near. The idiom "to carry someone in one's bosom" does not refer to carrying someone in the fold of one's garment but rather to embrace someone with one's arms, or to hold someone's identity and needs in one's heart, reason and conscious mind.
It's unclear where this word comes from, but since the Greek alphabet derives from the Hebrew one, this word too might very well be of Semitic origin. The root that jumps to mind is תור (tur), to explore in a circular motion. Verb תאר (ta'ar) means to outline or trace. Noun תאר (to'ar), means shape or form. Noun תור (tor), curiously enough, means dove, which gracefully ties Matthew 3:16 to Genesis 15:1. Another obvious similarity with our noun θωραξ (thorax) comes with the familiar word Torah, instructions.
Our noun is used 5 times in the New Testament, see full concordance, and has no derivations.