When someone says a trade is printing, they mean it is producing profit. Printing also has a literal meaning: a trade prints when it executes and appears on time-and-sales.
The slang usage tends to peak near market tops, when everything is printing and overconfidence is at its highest.
Example: the calls were printing all morning. Or: a 10,000-share block just printed at $52.10.
Related: time-and-sales, overconfidence, moon, rip