The phrase comes from blackjack, where doubling down is a defined bet with fixed odds. In trading it has no such structure; it just means the exposure is now twice what you originally judged appropriate.
The framing is always about conviction, and conviction is rarely the reason. A trader who genuinely believed more strongly would have to explain why the belief arrived only after the price fell.
If the second entry is real, it should be pre-planned with its own level and should keep total risk inside the normal limit. Otherwise it is adding-to-losers with a confident name.
Related: adding-to-losers, escalation-of-commitment, martingale-mindset, full-port