The story
From tug + boat. Sense 2 from monthly tugboat, a reference to the saying "our ship has come in," here referring to a small, "reliable" "tugboat" instead of a large windfall.
Etymology from Wiktionary, CC BY-SA 4.0.
tugboat
Today it means: A small, powerful boat used to push or pull barges or to help maneuver larger vessels. Full definition →