“Like a turd that won't flush” can mean an excrement that you can‘t flush down the toilet. But when said to someone or about someone, it means an irritating person that just won’t go away.

It is mostly used in the latter sense.
Example 1:

He just keeps on bugging everyone and doesn’t know when to go away, like a turd that won't flush.

Example 2:

Guy 1: I don’t want him to come with us!

Guy 2: So why don’t we ignore him, and he might take the hint.

Guy 1: I doubt it, he’s like a turd that won't flush.
by scooby007 January 16, 2011
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