What's the meaning of "seat pig", It's like bastard or sth?
I'll be appreciated if you can give me an answer.

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Usually refers to somebody who takes up extra room on public transportation so that other people can't sit next to them.

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I have not heard this either. But If i had to guess, it's probably similar to 'Couch Potato'.

Seat Pig: a person who has taken on the attributes of a pig because they are always seated.

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It's "seat pig", however, what is the meaning of "sit pig"?

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Do you mean "seat belt", or "Sit Pig!" ?

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I've never heard of it either. Is this something you read or something you heard? Do you have the rest of the sentence?