From Soteriology101:

The Hebrew word [verb] נדב naw-dab’ is a primitive root that means – to impel; hence, to volunteer (as a soldier), to present spontaneously…primarily translated as an adverb “willingly” which indicates free motivation or voluntary decision. It is used 17 times in 15 verses throughout OT Scripture [also 3 times in 3 verses using the same root in Aramaic – Ezra 7:13, 15, 16]. (Most of definitions for this paper are adapted from Strong’s Concordance lexical definitions.)

…

The noun נדבה ned-aw-baw’ is used 26 times in 25 verses, mostly in connection with a voluntary – “freewill” – offering to God. With all these verses one cannot help but ask “How can you have a freewill offering without a freewill?” Calvinists reject its normal meaning, but the Bible literally uses the word 26 times. Even the Calvinist translators of the KJV and ESV freely chose “freewill” as a suitable translation. Their translation choice is telling of what they believed this original word meant.

…

—-[from προθυμια proth-oo-mee’-ah, meaning predisposition. See also – 2Co 8:11, 12, 19, 9:2;] The Calvinist may endeavor to suggest this willing predisposition of the Bereans was a result of regeneration, which they think is before faith is expressed. It is very difficult to convince them otherwise when their loyalty to Calvinism is so strong that they refuse to see the gospel of John clearly teaches light is freely received before faith which is before new birth life is given. See John 1:4-13, 12:35-36, 20:30-31.