2 Samuel 19:24
And Mephibosheth the son of Saul came down to meet the king, and had neither dressed his feet, nor trimmed his beard, nor washed his clothes, from the day the king departed until the day he came again in peace. And Mephibosheth the son of Saul came down to meet the king, and had neither dressed his feet, nor trimmed his beard, nor washed his clothes, from the day the king departed until the day he came again in peace.
The verse belongs to the account of David's lament over Absalom and his return to Jerusalem in peace with victory after Absalom's rebellion and the death of the petted idol of the kingdom. It falls within the closing history running from the long war between the house of Saul and the house of David to David's victory over Absalom, and the chapter shows David's continuing kindness to Saul's household. That history of the reign of David shows the difficulties he encountered and the sad consequences of his many wives and of sons he was unable to control, seen in his family troubles and return to Jerusalem.
Those family troubles in David's later history are the retribution and judgments for his wicked conduct and wrongdoing toward Uriah, fulfilling Nathan's denunciation through all the rest of his life. The retribution came principally in his own family, in the sad history of Amnon and Tamar and in the rebellion, unnatural crimes, and death of Absalom, with even Adonijah's rebellion and death as part of it. This proves that what a man sows that he reaps, that wrongdoing brings back the same kind of evil upon the wrongdoer, and that sin never pays except in its own kind. The detailed faithfulness with which this history is told stamps the whole account with inspiration and shows that God does not hide the sins of even the greatest of kings.
Mephibosheth's neglected feet, untrimmed beard, and unwashed clothes from the day the king departed until the day he came again in peace were an expression of deep mourning while the king was away. His leaving the beard untrimmed for that mourning also shows that trimming the beard is not in itself sinful.