Albert Barnes Commentary 1 Kings 21:27

"And it came to pass, when Ahab heard those words, that he rent his clothes, and put sackcloth upon his flesh, and fasted, and lay in sackcloth, and went softly." — 1 Kings 21:27 (ASV)

Ahab's repentance resembles that of the Ninevites (Jonah 3:5). It has the same outward signs—fasting and sackcloth—and it has much the same inward character. It springs not from love or from hatred of sin, but from fear of the consequences of sin. Thus, although sincere and real while it lasts, it is shallow and exceedingly short-lived. However, to show His readiness to receive the sinner who turns to Him, God accepted the imperfect offering (just as He also accepted the repentance of the Ninevites) and allowed it to delay the execution of the sentence (1 Kings 21:29). Similarly, the repentance of the Ninevites postponed the fall of Nineveh for a century.

And lay in sackcloth — In this respect, he seems to have gone beyond the usual practice. We do not read elsewhere of mourners passing the night in sackcloth.

And went softly — “As if he had no heart to go about any business” (Patrick).