'Fast' in the Bible

Only take diligent heed to do the commandment and the law which Moses the servant of Yahweh commanded you, to love Yahweh your God, to walk in all his ways, to keep his commandments, to hold fast to him, and to serve him with all your heart and with all your soul."

but hold fast to Yahweh your God, as you have done to this day.

"But if you do at all go back, and hold fast to the remnant of these nations, even these who remain among you, and make marriages with them, and go in to them, and they to you;