John Gill Commentary Jeremiah 6:26

"O daughter of my people, gird thee with sackcloth, and wallow thyself in ashes: make thee mourning, as for an only son, most bitter lamentation; for the destroyer shall suddenly come upon us." — Jeremiah 6:26 (ASV)

O daughter of my people, gird thee with sackcloth
Either as a token of repentance for sin; so the king of Nineveh and his subjects did, to show their repentance, (Jonah 3:6Jonah 3:8) or as a sign of mourning, for the calamities coming on them, (Genesis 37:34)

and wallow thyself in ashes ;
or roll yourself in them, as a token of the same.

The Targum is, ``cover your heads with ashes.''

Make thee mourning as for an only son ;
which of all is the most bitter: and therefore it is added, most bitter lamentation ;
see (Zechariah 12:10) .

For the spoiler shall suddenly come upon us ;
namely, Nebuchadnezzar, that would spoil their cities, towns, villages, and houses, and them of all their wealth and substance, and carry it away.