Meaning:
“玩” literally means “to play,” “了” is a marker for perfect tense, “个” is a quantifier, and “寂寞” means “solitude.” This term, coined by Chinese netizens and literally translated as “playing with solitude,” is a playful way to say “coming to no results; unfruitful.” If people have tried hard to accomplish something but their efforts didn’t pay off, perhaps they will experience a sense of emptiness and solitude in the end.
Example:
A: 最近股市火爆，你赚钱了吗？
Zuìjìn gǔshì huǒbào，nǐ zhuànqián le ma？
The stock market has been bullish recently. Have you made a fortune?
B: 我买的股票都不赚钱，入市一个月，玩了个寂寞。
Wǒ mǎi de gǔpiào dōu bú zhuànqián，rùshì yī gè yuè，wán le gè jìmò。
The stocks I bought didn’t do well. It’s been a month since I jumped into the market, but my investments haven’t paid off.