As a straight male I've asked plenty of people, both male and female to grab a cup of coffee. So far no one seems to have been confused. They all understood it was just getting coffee and chatting.

Ok, then what's the code for cup of coffee?

The same. The only reason plausible deniability works is that it's ambiguous.

Ambiguous might not be quite the right term. In many cases for such an invite, it's desirable that it be pretty clear you mean a date, but you still retain deniability.

I guess you could say you want the implication to be clear and fairly unambiguous, but not explicit/provable.

Double entendre comes to mind.

"Hey, I'm heading out for coffee in 15, want to join?"

There is a big difference between "lets meet and talk to each other for an hour at the coffee shop on the weekend" and "We are at work, lets take a quick break!"

"Cup of coffee" -- it just depends on the context and how it is asked.

"Want to grab some (Starbucks|Blue Bottle|Peetz|etc)?" Maybe they'll just get a cookie.