Cumshaw was, basically, anything that was obtained outside of normal channels, whether repair work done without a 2-Kilo or stuff that was obtained without a supply chit.
And it could be almost any way imaginable.
This is no shit:
A young lieutenant junior grade was standing watch as the Command Duty Officer of a ship that had just entered drydock a few days previously. A sister ship was also in the shipyard, getting ready to sail on sea trials. A lieutenant and four sailors came onto the quarterdeck from the second ship well after evening chow and asked to speak to the CDO. When the CDO appeared, the lieutenant pronounced that he was there to take one of the legs to the forward kingpost.
The kingpost was a gizmo that was erected on the forecastle during an underway replenishment. The spanwire from the cargo ship would be connected to it. The bottom of the kingpost bolted to a heavy baxter bolt. The outboard side of the top of the kingpost connected to two legs, which in turn were bolted to baxter bolts in the deck. The kingpost and its legs were made of heavy and high-grade aluminum piping.
All of those parts were original issue to the ships. Replacements were scarcer than honest politicians (and almost as expensive as crooked ones). For whatever reason, the soon-to-sail ship was missing one. It would be a serious ding to not be able to take on supplies at both unrep stations.
The CDO stroked his chin and said: "I don't know anything about you taking the kingpost leg. Nobody told me about this."
The lieutenant said that it was all arranged between the two ship's Weapons Officers and that he needed it, now.
The CDO asked when it had been arranged and he was told "two days ago, I think."
The CDO looked quizzical and said: "I have a tough decision to make, sir. Maybe you can help me."
Now the lieutenant looked puzzled: "If I can."
"Weps is on TDY at a school. So your Weps would have had to talk to our acting Weps. Which is me. And I've not talked to anybody about this. So this is my dilemma: Either I shoot you or arrest you for attempted theft. If you're not offa my fucking ship in ten seconds, I'm gonna choose one or the other." He then turned to the Petty Officer of the Watch and said: "Gimme your sidearm and a magazine."
The lieutenant and his men got off the ship as fast as they could throw out the required salutes.