Yes the calvinist definition came from the reformation, not the Bible.
Thayer's Greek Lexicon
STRONGS NT 2434: ἱλασμός
Strong's Exhaustive Concordance
propitiation, sin offering
Atonement, i.e. (concretely) an expiator -- propitiation.
ἱλασμός, ἱλασμοῦ, ὁ (ἱλάσκομαι);
1. an appeasing, propitiating, Vulg.propitiatio (Plutarch, de sera num. vind. c. 17; plural joined with καθαρμοι, Plutarch, Sol. 12; with the genitive of the object τῶν θεῶν, the Orphica Arg. 39; Plutarch, Fab. 18; θεῶν μῆνιν ἱλασμοῦ καί χαριστηριων δεομένην, vit. Camill. 7 at the end; ποιεῖσθαι ἱλασμόν, of a priest offering an expiatory sacrifice, 2 Macc. 3:33).
2. in Alex. usage the means of appeasing, a propitiation: Philo, alleg. leg. 3: § 61; προσοίσουσιν ἱλασμόν, for חַטָּאת,
Ezekiel 44:27; περί τῶν ἁμαρτιῶν, of Christ,
1 John 2:2;
1 John 4:10 (κριός τοῦ ἱλασμοῦ,
Numbers 5:8; (cf. ἡμέρα τοῦ ἱλασμοῦ,
Leviticus 25:9); also for סְלִיחָה, forgiveness,
Psalm 129:4 ();
Daniel 9:9, Theod.). (Cf. Trench, § lxxvii.)
What is expiation or atonement?
"Atonement" is to attain forgiveness for some sin or transgression, e.g. "I atoned for betraying my friend". It is something that you do.
"Expiation" means to clear away the record, to make it as if it never existed. It is not something you can do - you cannot expiate your own sins
hope this helps !!!