Tā zìchēng dānshēngǒu, qíshí tǐng xiǎngshòu yí gè rén de shēnghuó.
He calls himself a 'single dog', but he actually enjoys living alone.
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Internet slang, almost always self-deprecating. Acceptable to say about yourself or close friends who are in on the joke — calling someone else a 单身狗 unprompted can sound rude.