Abarim Publications' online Biblical Hebrew Dictionary
The verb פלש (palash) indicates a rather hysterical action, induced by grief and involving dust or ashes to roll around in. It occurs in the Bible a mere four times: Micah 1:10, Ezekiel 27:30, Jeremiah 6:26 and Jeremiah 25:34.
BDB Theological Dictionary hints at similar verbs in cognate languages, which reveal the more fundamental meaning of to break into or through, dig a hole or burrow.
Some scholars have proposed that this verb's fundamental meaning is to roll (around in ashes, for instance). Perhaps this verb reflects the act of diving into a heap of ashes, or perhaps it reflects the feeling of being pierced though by some great grief. Perhaps it does all of the above.