Abarim Publications' online Biblical Greek Dictionary
The identical word κορος (koros), also spelled κουρος (kouros), means son in the sense of boy or lad, but isn't used as such in the Bible (apart from in the name Dioscuri). An identically spelled word means satiety or surfeit in Homer and the consequence of it, insolence, in Pindar and others (perhaps related to κορεννυμι, korennumi, to satisfy: Acts 27:38, 1 Corinthians 4:8). On rare occasions this word may even denote a broom.
Combined with the prefix επι (epi), meaning on or onto, this word creates the noun επικουρος (epikouros), which denotes auxiliary troops as opposed to the military force formed from πολιτης (polites), civilians. In other words: the elite army consisted of the "sons", and the επικουρος (epikouros) were the "for-the-sons" or "in addition to the sons"
This word also does not occur in the Bible (apart from in the name Epicurean), but the derived noun επικουρια (epikouria), meaning help (from auxiliary, non-native troops) occurs quite tellingly in Acts 26:22, where Paul declares to have been given epikouria (that's help from non-natives or gentiles) from God.