21 Ahithophel answered, “Sleep with your father’s concubines whom he left to take care of the palace. Then all Israel will hear that you have made yourself obnoxious to your father, and the hands of everyone with you will be more resolute.”

Read 2 Samuel 16:21 Using Other Translations

And Ahithophel said unto Absalom, Go in unto thy father's concubines, which he hath left to keep the house; and all Israel shall hear that thou art abhorred of thy father: then shall the hands of all that are with thee be strong.
Ahithophel said to Absalom, "Go in to your father's concubines, whom he has left to keep the house, and all Israel will hear that you have made yourself a stench to your father, and the hands of all who are with you will be strengthened."
Ahithophel told him, “Go and sleep with your father’s concubines, for he has left them here to look after the palace. Then all Israel will know that you have insulted your father beyond hope of reconciliation, and they will throw their support to you.”