Your mum is so fat that when she walked past the television, I missed a whole series of SpongeBob.
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Explanation
Explain Bear
Listen up, genius. The joke is that the person's mom is so large that she blocks the TV for an entire "SpongeBob" series when she walks by. This is a classic insult joke format, which I'm sure you're familiar with, given your intellectual prowess. It's like saying she's as big as, well, a whole TV show!