Douglas Harper's Etymology Dictionary
1850s nautical slang, perhaps from British naval phrase commodity bag.
Wiktionary
n. 1 A small bag used to carry one's personal effects or toiletries while traveling. 2 A sailor's small bag to hold thread, needles, tape, etc.; a housewife.
WordNet
n. kit used by sailors and soldiers
Usage examples of "ditty bag".
And then, as though in answer to the frantic thought, a man stepped into view below, and casually dropped his coat over the ditty bag.
His sea chest and ditty bag lay forlorn on the bare cabin deck, pathetic indications of his immediate departure.
I have stuffed my wig into the ditty bag already, waiting for her.
Everything he was taking was in his pack or attached to his gear webbing, both of which, along with his helmet, were in a ditty bag under his rack.
The ditty bag was necessary, because the pack and webbing were chameleoned but the bag wasn't--he wouldn't have to grope to find his gear when it was time to move out.
Initials carved in a bulkhead, old hooks where a hammock or ditty bag had been hung.
Perhaps the lab could use it to tell where in the park the bag had been before it was shanghaied into service as a ditty bag for the deceased.
She stepped out of the cabin, turned to the right, and shouldered the ditty bag.
He also had a canvas ditty bag loaded with film packs and flashbulbs hanging from his shoulder.
Casey needed some big kind of magic right now to pull this off, and she wasn't sure Juana's little ditty bag contained it!
She concentrated on tossing a few last possessions into a ditty bag.