1: στῆθος
(Strong's #4738 — Noun Neuter — stethos — stay'-thos )

connected with histemi, "to stand," i.e., that which stands out, is used of mourners in smiting the "breast," Luke 18:13; 23:48; of John in reclining on the "breast" of Christ, John 13:25; 21:20; of the "breasts" of the angels in Revelation 15:6 .

2: μαστός
(Strong's #3149 — Noun Masculine — mastos — mas-tos' )

used in the plural, "paps," Luke 11:27; 23:29; Revelation 1:13 , AV, is preferably rendered "breasts," in the RV.