A "Man O' War" was a British Royal Navy expression for a powerful warship or frigate from the 16th to the 19th century.
There is also a Scottish country dance called British Man O' War.
Pretty Susan fell a-weeping - "Young sailor," she did say,
"How can you be so venturesome, to throw your life away;
When I am twenty one I shall receive my store,
Jolly sailor, do not venture in a British Man o' War."
"Oh, Susan, lovely Susan, the truth to you I'll tell,
The British flag's insulted, old England knows it well;
I may be crowned with laurels, so, like a jolly tar,
I will face the wars of China in a British Man o' War."
"Oh, sailor, do not venture to face the proud Chinese,
For they will prove more treacherous than any Portuguese,
All by some deadly dagger you may receive a scar,
So turn your inclination from a British Man o' War."
"Oh, Susan, lovely Susan, the time will quickly pass,
Now come down to the ferry house and take a parting glass;
My shipmate is there waiting to row me from the shore,
And it's for old England's glory in a British Man o' War."
The sailor took his handkerchief and cut it fair in two -
"Oh, Susan, keep one half for me, and I'll do the same for you;
Though bullets may surround me, and cannons loudly roar,
I will fight for fame and Susan in a British Man o' War."