(n.) someone who is more likely to dispute an order given to them than act on it
When you tell someone to do something and instead they argue against it or dispute the task being given to them, then they’re a sea-lawyer. That’s a term that can also be used more generally to refer to a particularly eloquent or convincing yet utterly obstinate and argumentative person.
Why sea-lawyer? This expression first emerged in nineteenth century maritime slang, and was explained in a contemporary dictionary of such as:
An idle litigious long-shorer, more given to question orders than to obey them. One of the pests of the navy as well as of the mercantile marine.
William Henry Smyth, The Sailor’s Word-Book (Vol. 1, 1867)
The reference to a lawyer here is clear: the phrase is a witty allusion to a lawyer’s habit of questioning every detail and eventuality of a legal case. But more puzzling here is an even earlier use of this word, recorded in another nineteenth century dictionary:
Sea Lawyer: a shark.
Lexicon Balatronicum (1811)
Why were sharks—and as it happens, in particular tiger sharks—also known as sea-lawyers? Apparently it’s a reference to their cunningness, sharp minds, and an in-built perilousness that defies their appearance:
The vitality of the shark is remarkable. After being mangled and apparently killed, it seems to possess the power of doing injury. While lying as if dead on the deck of a vessel, its jaws will make a sudden snap at anything near it. Acquainted with these unlooked-for and deadly proceedings, the sailors jocularly call the shark a ‘sea-lawyer’.
Chambers’ Home Book or, Pocket Miscellany (Vol. 4, 1853)
So as another Victorian guide to all of this explains, in conclusion:
An argumentative sailor is a “sea-lawyer,” the brown shark is a “sea-attorney,” and a lawyer is a “land shark.”
‘Curiosities of Nautical Nomenclature’, in The United Service (1889)