the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
Allah says: "And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what appears thereof; that they should draw their head coverings over their bosoms…" [Sûrah al-Nûr: 31]
This verse is actually stronger evidence for covering the bosom than it is for covering the head, since the term "headscarf" (khimâr) is mentioned incidentally, while the command to cover the bosom with that headscarf is a direct, explicit command in the Qur'an.
The Arabic word here is juyûb (plural of jayb) which is used in Arabic to mean the bosom. Its original meaning was the slit in the front of a shirt or garment that allowed the head to go through and that would consequently expose the breasts. It then took on the meaning of bosom according to the customs of speech.
And Allah knows best.
Source: Islam Today