I think the term originates from Hong Kong. The saying that giving birth to a Char Siew is better than a useless child, at least the mother can eat the Char Siew.

However in this /k, the meaning can deviate away from the original. I take it that most /k are not very good in English.

There are other words that deviated in /k are:
1. Drill (photo). The actual sentence should be you know the drill (of posting a photo)
2. Cool story. I still don't know what the hell it means but I can understand /k meaning should be unbelievable story.
3. Kuda. I think it is attention seeking.