This answer is intended as a follow-up to fdb’s answer, with which I basically agree.
Is it a Greek-ism?
Yes. Atticism might be another appropriate word. As mentioned, the phrase of interest is ἄνδρες ἀδελφοί (andres adelphoi; men, brothers). This appears to be modeled on the typical Athenian oratorical introductory formula, andres Athenaioi ("men, Athenians” or “men of Athens”).1 Luke records other related phrases using ἄνδρες in apposition to a more specific identifier. These include:
- ἄνδρες Γαλιλαῖοι (1:11, “men of Galilee”);
- ἄνδρες Ἰουδαῖοι (2:14, “men of Judea”);
- Ανδρες Ἰσραηλῖται (2:22, “men of Israel”);
- ἄνδρες Ἀθηναῖοι (17:22, “men of Athens”).
The phrase ἄνδρες ἀδελφοί is an adaptation of the classical formula that introduces ἀδελφοί, a term commonly used among early Christians to refer to each other, as recorded throughout the New Testament.
Does this imply that their audience was entirely composed of men, or were women there as well?
As mentioned in other answers, ἀνήρ, (the dictionary form of ἄνδρες) is more specific for “maleness” than ἄνθρωπος, another word often translated as “man”. From BDAG, ἀνήρ (bold/italics original):
an adult human male, man, husband....a. in contrast to a woman....b. in contrast to a boy
For extensive discussion in support of the idea that men in particular are in mind when this word is used, see Wayne Grudem.2 ("This word never refers to women.") For interaction with and objections to Dr. Grudem’s argument, see D.A. Carson, The Inclusive-Language Debate: A Plea for Realism.3 He agrees with the basic point:
one should assume [ἄνδρες] refers to male human beings, unless there is convincing contextual counter-evidence.
However, he believes Grudem’s statement above is demonstrably too strong (p. 159). The commonly adduced counter-example is drawn from Acts 17:34 where it is clear within the verse that the referent includes at least one woman:
τινὲς δὲ ἄνδρες κολληθέντες αὐτῷ ἐπίστευσαν, ἐν οἷς καὶ Διονύσιος ὁ Ἀρεοπαγίτης καὶ γυνὴ ὀνόματι Δάμαρις καὶ ἕτεροι σὺν αὐτοῖς.
But some men joined him and believed, among whom also were Dionysius the Areopagite and a woman named Damaris and others with them.
Thus, it is possible but unusual for ἄνδρες to denote a group that includes women. Commentators as early as Chrysostom (I believe a native Greek speaker) interpreted ἄνδρες ἀδελφοί in Acts as gender-neutral.4 Every modern commentary I was able to review agreed.1,5,6 Quoting F.F. Bruce:
the word is otiose, and does not necessarily exclude women.
1. Joseph A. Fitzmyer. The Acts of the Apostles: A New Translation with Introduction and Commentary AYB; New Haven: Yale University Press, 1974.
2. Reproduced at the linked page from: Vern S. Poythress and Wayne A. Grudem The Gender-Neutral Bible Controversy by (Nashville: Broadman and Holman, 2000), pp. 321-333.
3. D. A. Carson. The Inclusive-Language Debate: A Plea for Realism. Grand Rapids: Baker, 1998. Out of print and available in full at the link.
4. John Chrysostom. Homilies on the Acts of the Apostles and the Epistle to the Romans. Ed. Philip Schaff. T&T Clark, Edinburgh.
5. F.F. Bruce. The Acts of the Apostles: Greek text with introduction and commentary. Eerdmans, 1990. Quote here from p. 108.
6. David Peterson, The Acts of the Apostles. PNTC; Eerdmans, 2009.