Deuteronomy 1:43 (ESV)

43 So I spoke to you, and you would not listen; but you rebelled against the command of the LORD and presumptuously went up into the hill country.

Moses had warned the people, but they ignored him, and instead rebelled also against this latest command. They attempted to go up without God, which was an act of great rebelliousness. They acted presumptuously, that is, with brash arrogance, against the Lord.