In many ahadith, the idiom "ثكلتك أمك" ("May your mother lose you") is found; I have also seen similar constructs with different wording (e.g. "ثكلتك الثواكل"). From context, this appears to be used as an admonition.
What exactly does this idiom mean, and how is it intended to be used? Any details on how it originated would also be useful, especially if it's Islamically-based.
Athakul (الثكل) means to lose, and it refers to a women losing her child. Athakool (الثكول) is the/a women who has lost her child. As for the statement Thakilatka Ummuk (ثكلتك أمك) pronounced with a Kasra on the kaaf of ثكلتك, it is a statement/dua which isn't really wanted, for example, saying something that you don't really mean, joking. This statement is said to show resentment, to give discipline, to warn one from inattention or ignorance, and to make great a situation.
Sources: Lisanul Arab and Awqaf.ae
It's an old arabic phrase as you said, it's somehow slang. When you find someone doing weird/dangerous/wrong deed and you try to stop him with a shout "You're a dead man!"
It's very similar to that.
