I've come across the term of a tainted coin several times now. I understand that coins, or better, the origin of a coin can be traced throughout the blockchain.
I've read that bitcoins can be disassociated from an address, by sending it to a sufficiently large enough wallet which handles many transactions, thus making a transactions origin "untraceable".
This collides with my understanding of the blockchain, where each transaction can be tracked back to its origin.
What does the concept of a tainted coin mean in this context and how can a coins taint be measured?